Data Science

VERSION 3: AN INTRODUCTION TO

Jeffrey Stanton, Syracuse University

(With A Contribution By Robert W. De Graaf)

© 2012, 2013 By Jeffrey Stanton, !

Portions © 2013, By Robert De Graaf

This book is distributed under the Creative Commons Attribution-

NonCommercial-ShareAlike 3.0 license. You are free to copy, dis-

tribute, and transmit this work. You are free to add or adapt the

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This book was developed for the Certiﬁcate of Data Science pro-

gram at Syracuse University’s School of Information Studies. If

you ﬁnd errors or omissions, please contact the author, Jeffrey Stan-

ton, at jmstanto@syr.edu. A PDF version of this book and code ex-

amples used in the book are available at:

http://jsresearch.net/wiki/projects/teachdatascience

The material provided in this book is provided "as is" with no war-

ranty or guarantees with respect to its accuracy or suitability for

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Thanks to Ashish Verma for help with revisions to Chapter 10!

i

INTRODUCTION TO DATA SCIENCE

ii

Data Science: Many Skills

Data Science refers to an emerging area of work concerned with the collection, preparation, analysis,

visualization, management, and preservation of large collections of information. Although the name

Data Science seems to connect most strongly with areas such as databases and computer science,

many different kinds of skills - including non-mathematical skills - are needed.

SECTION 1

For some, the term "Data Science" evokes images

of statisticians in white lab coats staring ﬁxedly

at blinking computer screens ﬁlled with scrolling

numbers. Nothing could be further from the

truth. First of all, statisticians do not wear lab

coats: this fashion statement is reserved for biolo-

gists, doctors, and others who have to keep their

clothes clean in environments ﬁlled with unusual

ﬂuids. Second, much of the data in the world is

non-numeric and unstructured. In this context,

unstructured means that the data are not ar-

ranged in neat rows and columns. Think of a

web page full of photographs and short mes-

sages among friends: very few numbers to work

Overview

1. Data science includes data

analysis as an important

component of the skill set

required for many jobs in

this area, but is not the only

necessary skill.

2. A brief case study of a

supermarket point of sale

system illustrates the many

challenges involved in data

science work.

3. Data scientists play active

roles in the design and

implementation work of

four related areas: data

architecture, data

acquisition, data analysis,

and data archiving.

4. Key skills highlighted by the

brief case study include

communication skills, data

analysis skills, and ethical

reasoning skills.

Word frequencies from the deﬁnitions in a Shakespeare glossary. While professional data scientists do need

skills with mathematics and statistics, much of the data in the world is unstructured and non-numeric.

Data Science: Many Skills

3

with there. While it is certainly true that companies, schools, and

governments use plenty of numeric information - sales of prod-

ucts, grade point averages, and tax assessments are a few examples

- there is lots of other information in the world that mathemati-

cians and statisticians look at and cringe. So, while it is always use-

ful to have great math skills, there is much to be accomplished in

the world of data science for those of us who are presently more

comfortable working with words, lists, photographs, sounds, and

other kinds of information.

In addition, data science is much more than simply analyzing data.

There are many people who enjoy analyzing data and who could

happily spend all day looking at histograms and averages, but for

those who prefer other activities, data science offers a range of

roles and requires a range of skills. Let’s consider this idea by think-

ing about some of the data involved in buying a box of cereal.

Whatever your cereal preferences - fruity, chocolaty, ﬁbrous, or

nutty - you prepare for the purchase by writing "cereal" on your

grocery list. Already your planned purchase is a piece of data, al-

beit a pencil scribble on the back on an envelope that only you can

read. When you get to the grocery store, you use your data as a re-

minder to grab that jumbo box of FruityChocoBoms off the shelf

and put it in your cart. At the checkout line the cashier scans the

barcode on your box and the cash register logs the price. Back in

the warehouse, a computer tells the stock manager that it is time to

request another order from the distributor, as your purchase was

one of the last boxes in the store. You also have a coupon for your

big box and the cashier scans that, giving you a predetermined dis-

count. At the end of the week, a report of all the scanned manufac-

turer coupons gets uploaded to the cereal company so that they

can issue a reimbursement to the grocery store for all of the coupon

discounts they have handed out to customers. Finally, at the end of

the month, a store manager looks at a colorful collection of pie

charts showing all of the different kinds of cereal that were sold,

and on the basis of strong sales of fruity cereals, decides to offer

more varieties of these on the store’s limited shelf space next

month.

So the small piece of information that began as a scribble on your

grocery list ended up in many different places, but most notably on

the desk of a manager as an aid to decision making. On the trip

from your pencil to manager’s desk, the data went through many

transformations. In addition to the computers where the data may

have stopped by or stayed on for the long term, lots of other pieces

of hardware - such as the barcode scanner - were involved in col-

lecting, manipulating, transmitting, and storing the data. In addi-

tion, many different pieces of software were used to organize, ag-

gregate, visualize, and present the data. Finally, many different "hu-

man systems" were involved in working with the data. People de-

cided which systems to buy and install, who should get access to

what kinds of data, and what would happen to the data after its im-

mediate purpose was fulﬁlled. The personnel of the grocery chain

and its partners made a thousand other detailed decisions and ne-

gotiations before the scenario described above could become real-

ity.

Obviously data scientists are not involved in all of these steps.

Data scientists don’t design and build computers or barcode read-

ers, for instance. So where would the data scientists play the most

valuable role? Generally speaking, data scientists play the most ac-

tive roles in the four A’s of data: data architecture, data acquisition,

4

data analysis, and data archiving. Using our cereal example, let’s

look at them one by one. First, with respect to architecture, it was

important in the design of the "point of sale" system (what retailers

call their cash registers and related gear) to think through in ad-

vance how different people would make use of the data coming

through the system. The system architect, for example, had a keen

appreciation that both the stock manager and the store manager

would need to use the data scanned at the registers, albeit for some-

what different purposes. A data scientist would help the system ar-

chitect by providing input on how the data would need to be

routed and organized to support the analysis, visualization, and

presentation of the data to the appropriate people.

Next, acquisition focuses on how the data are collected, and, impor-

tantly, how the data are represented prior to analysis and presenta-

tion. For example, each barcode represents a number that, by itself,

is not very descriptive of the product it represents. At what point

after the barcode scanner does its job should the number be associ-

ated with a text description of the product or its price or its net

weight or its packaging type? Different barcodes are used for the

same product (for example, for different sized boxes of cereal).

When should we make note that purchase X and purchase Y are

the same product, just in different packages? Representing, trans-

forming, grouping, and linking the data are all tasks that need to

occur before the data can be proﬁtably analyzed, and these are all

tasks in which the data scientist is actively involved.

The analysis phase is where data scientists are most heavily in-

volved. In this context we are using analysis to include summariza-

tion of the data, using portions of data (samples) to make infer-

ences about the larger context, and visualization of the data by pre-

senting it in tables, graphs, and even animations. Although there

are many technical, mathematical, and statistical aspects to these

activities, keep in mind that the ultimate audience for data analysis

is always a person or people. These people are the "data users" and

fulﬁlling their needs is the primary job of a data scientist. This

point highlights the need for excellent communication skills in

data science. The most sophisticated statistical analysis ever devel-

oped will be useless unless the results can be effectively communi-

cated to the data user.

Finally, the data scientist must become involved in the archiving of

the data. Preservation of collected data in a form that makes it

highly reusable - what you might think of as "data curation" - is a

difﬁcult challenge because it is so hard to anticipate all of the fu-

ture uses of the data. For example, when the developers of Twitter

were working on how to store tweets, they probably never antici-

pated that tweets would be used to pinpoint earthquakes and tsu-

namis, but they had enough foresight to realize that "geocodes" -

data that shows the geographical location from which a tweet was

sent - could be a useful element to store with the data.

All in all, our cereal box and grocery store example helps to high-

light where data scientists get involved and the skills they need.

Here are some of the skills that the example suggested:

•

Learning the application domain - The data scientist must

quickly learn how the data will be used in a particular context.

•

Communicating with data users - A data scientist must possess

strong skills for learning the needs and preferences of users.

Translating back and forth between the technical terms of com-

5

puting and statistics and the vocabulary of the application do-

main is a critical skill.

•

Seeing the big picture of a complex system - After developing an

understanding of the application domain, the data scientist must

imagine how data will move around among all of the relevant

systems and people.

•

Knowing how data can be represented - Data scientists must

have a clear understanding about how data can be stored and

linked, as well as about "metadata" (data that describes how

other data are arranged).

•

Data transformation and analysis - When data become available

for the use of decision makers, data scientists must know how to

transform, summarize, and make inferences from the data. As

noted above, being able to communicate the results of analyses

to users is also a critical skill here.

•

Visualization and presentation - Although numbers often have

the edge in precision and detail, a good data display (e.g., a bar

chart) can often be a more effective means of communicating re-

sults to data users.

•

Attention to quality - No matter how good a set of data may be,

there is no such thing as perfect data. Data scientists must know

the limitations of the data they work with, know how to quan-

tify its accuracy, and be able to make suggestions for improving

the quality of the data in the future.

•

Ethical reasoning - If data are important enough to collect, they

are often important enough to affect people’s lives. Data scien-

tists must understand important ethical issues such as privacy,

and must be able to communicate the limitations of data to try to

prevent misuse of data or analytical results.

The skills and capabilities noted above are just the tip of the ice-

berg, of course, but notice what a wide range is represented here.

While a keen understanding of numbers and mathematics is impor-

tant, particularly for data analysis, the data scientist also needs to

have excellent communication skills, be a great systems thinker,

have a good eye for visual displays, and be highly capable of think-

ing critically about how data will be used to make decisions and

affect people’s lives. Of course there are very few people who are

good at all of these things, so some of the people interested in data

will specialize in one area, while others will become experts in an-

other area. This highlights the importance of teamwork, as well.

In this Introduction to Data Science eBook, a series of data prob-

lems of increasing complexity is used to illustrate the skills and ca-

pabilities needed by data scientists. The open source data analysis

program known as "R" and its graphical user interface companion

"R-Studio" are used to work with real data examples to illustrate

both the challenges of data science and some of the techniques

used to address those challenges. To the greatest extent possible,

real datasets reﬂecting important contemporary issues are used as

the basis of the discussions.

No one book can cover the wide range of activities and capabilities

involved in a ﬁeld as diverse and broad as data science. Through-

out the book references to other guides and resources provide the

interested reader with access to additional information. In the open

source spirit of "R" and "R Studio" these are, wherever possible,

web-based and free. In fact, one of guides that appears most fre-

6

quently in these pages is "Wikipedia," the free, online, user sourced

encyclopedia. Although some teachers and librarians have legiti-

mate complaints and concerns about Wikipedia, and it is admit-

tedly not perfect, it is a very useful learning resource. Because it is

free, because it covers about 50 times more topics than a printed en-

cyclopedia, and because it keeps up with fast moving topics (like

data science) better than printed encyclopedias, Wikipedia is very

useful for getting a quick introduction to a topic. You can’t become

an expert on a topic by only consulting Wikipedia, but you can cer-

tainly become smarter by starting there.

Another very useful resource is Khan Academy. Most people think

of Khan Academy as a set of videos that explain math concepts to

middle and high school students, but thousands of adults around

the world use Khan Academy as a refresher course for a range of

topics or as a quick introduction to a topic that they never studied

before. All of the lessons at Khan Academy are free, and if you log

in with a Google or Facebook account you can do exercises and

keep track of your progress.

At the end of each chapter of this book, a list of Wikipedia sources

and Khan Academy lessons (and other resources too!) shows the

key topics relevant to the chapter. These sources provide a great

place to start if you want to learn more about any of the topics that

chapter does not explain in detail.

Obviously if you are reading this book you probably have access to

an appropriate reader app, probably on an iPad or other Apple de-

vice. You can also access this book as a PDF on the book’s website:

http://jsresearch.net/wiki/projects/teachdatascience/Teach_Data

_Science.html. It is valuable to have access to the Internet while

you are reading, so that you can follow some of the many links this

book provides. Also, as you move into the sections in the book

where open source software such as the R data analysis system is

used, you will sometimes need to have access to a desktop or lap-

top computer where you can run these programs.

One last thing: The book presents topics in an order that should

work well for people with little or no experience in computer sci-

ence or statistics. If you already have knowledge, training, or expe-

rience in one or both of these areas, you should feel free to skip

over some of the introductory material and move right into the top-

ics and chapters that interest you most. There’s something here for

everyone and, after all, you can’t beat the price!

Sources

http://en.wikipedia.org/wiki/E-Science

http://en.wikipedia.org/wiki/E-Science_librarianship

http://en.wikipedia.org/wiki/Wikipedia:Size_comparisons

http://en.wikipedia.org/wiki/Statistician

http://en.wikipedia.org/wiki/Visualization_(computer_graphics)

http://www.khanacademy.org/

http://www.r-project.org/

http://www.readwriteweb.com/hack/2011/09/unlocking-big-dat

a-with-r.php

http://rstudio.org/

7

Data comes from the Latin word, "datum," meaning a "thing given." Although the term "data" has

been used since as early as the 1500s, modern usage started in the 1940s and 1950s as practical

electronic computers began to input, process, and output data. This chapter discusses the nature of

data and introduces key concepts for newcomers without computer science experience.

CHAPTER 1

8

About Data

The inventor of the World Wide Web, Tim Berners-Lee, is often

quoted as having said, "Data is not information, information is not

knowledge, knowledge is not understanding, understanding is not

wisdom." This quote suggests a kind of pyramid, where data are

the raw materials that make up the foundation at the bottom of the

pile, and information, knowledge, understanding and wisdom rep-

resent higher and higher levels of the pyramid. In one sense, the

major goal of a data scientist is to help people to turn data into in-

formation and onwards up the pyramid. Before getting started on

this goal, though, it is important to have a solid sense of what data

actually are. (Notice that this book treats the word "data" as a plu-

ral noun - in common usage you may often hear it referred to as

singular instead.) If you have studied computer science or mathe-

matics, you may ﬁnd the discussion in this chapter a bit redun-

dant, so feel free to skip it. Otherwise, read on for an introduction

to the most basic ingredient to the data scientist’s efforts: data.

A substantial amount of what we know and say about data in the

present day comes from work by a U.S. mathematician named

Claude Shannon. Shannon worked before, during, and after World

War II on a variety of mathematical and engineering problems re-

lated to data and information. Not to go crazy with quotes, or any-

thing, but Shannon is quoted as having said, "The fundamental

problem of communication is that of reproducing at one point ei-

ther exactly or approximately a message selected at another point."

This quote helpfully captures key ideas about data that are impor-

tant in this book by focusing on the idea of data as a message that

moves from a source to a recipient. Think about the simplest possi-

ble message that you could send to another person over the phone,

via a text message, or even in person. Let’s say that a friend had

asked you a question, for example whether you wanted to come to

their house for dinner the next day. You can answer yes or no. You

can call the person on the phone, and say yes or no. You might

have a bad connection, though, and your friend might not be able

to hear you. Likewise, you could send them a text message with

your answer, yes or no, and hope that they have their phone

turned on so that they can receive the message. Or you could tell

your friend face to face, hoping that she did not have her earbuds

turned up so loud that she couldn’t hear you. In all three cases you

have a one "bit" message that you want to send to your friend, yes

or no, with the goal of "reducing her uncertainty" about whether

you will appear at her house for dinner the next day. Assuming

that message gets through without being garbled or lost, you will

have successfully transmitted one bit of information from you to

her. Claude Shannon developed some mathematics, now often re-

ferred to as "Information Theory," that carefully quantiﬁed how

bits of data transmitted accurately from a source to a recipient can

reduce uncertainty by providing information. A great deal of the

computer networking equipment and software in the world today

- and especially the huge linked worldwide network we call the

Internet - is primarily concerned with this one basic task of getting

bits of information from a source to a destination.

Once we are comfortable with the idea of a "bit" as the most basic

unit of information, either "yes" or "no," we can combine bits to-

gether to make more complicated structures. First, let’s switch la-

bels just slightly. Instead of "no" we will start using zero, and in-

stead of "yes" we will start using one. So we now have a single

digit, albeit one that has only two possible states: zero or one

(we’re temporarily making a rule against allowing any of the big-

ger digits like three or seven). This is in fact the origin of the word

"bit," which is a squashed down version of the phrase "Binary

9

digIT." A single binary digit can be 0 or 1, but there is nothing stop-

ping us from using more than one binary digit in our messages.

Have a look at the example in the table below:

Here we have started to use two binary digits - two bits - to create

a "code book" for four different messages that we might want to

transmit to our friend about her dinner party. If we were certain

that we would not attend, we would send her the message 0 0. If

we deﬁnitely planned to attend we would send her 1 1. But we

have two additional possibilities, "Maybe" which is represented by

0 1, and "Probably" which is represented by 1 0. It is interesting to

compare our original yes/no message of one bit with this new

four-option message with two bits. In fact, every time you add a

new bit you double the number of possible messages you can send.

So three bits would give eight options and four bits would give 16

options. How many options would there be for ﬁve bits?

When we get up to eight bits - which provides 256 different combi-

nations - we ﬁnally have something of a reasonably useful size to

work with. Eight bits is commonly referred to as a "byte" - this

term probably started out as a play on words with the word bit.

(Try looking up the word "nybble" online!) A byte offers enough dif-

ferent combinations to encode all of the letters of the alphabet, in-

cluding capital and small letters. There is an old rulebook called

"ASCII" - the American Standard Code for Information Interchange

- which matches up patterns of eight bits with the letters of the al-

phabet, punctuation, and a few other odds and ends. For example

the bit pattern 0100 0001 represents the capital letter A and the next

higher pattern, 0100 0010, represents capital B. Try looking up an

ASCII table online (for example, http://www.asciitable.com/) and

you can ﬁnd all of the combinations. Note that the codes may not

actually be shown in binary because it is so difﬁcult for people to

read long strings of ones and zeroes. Instead you may see the

equivalent codes shown in hexadecimal (base 16), octal (base 8), or

the most familiar form that we all use everyday, base 10. Although

you might remember base conversions from high school math

class, it would be a good idea to practice this a little bit - particu-

larly the conversions between binary, hexadecimal, and decimal

(base 10). You might also enjoy Vi Hart’s "Binary Hand Dance"

video at Khan Academy (search for this at

http://www.khanacademy.org or follow the link at the end of the

chapter). Most of the work we do in this book will be in decimal,

but more complex work with data often requires understanding

hexadecimal and being able to know how a hexadecimal number,

like 0xA3, translates into a bit pattern. Try searching online for "bi-

nary conversion tutorial" and you will ﬁnd lots of useful sites.

Combining Bytes into Larger Structures

Now that we have the idea of a byte as a small collection of bits

(usually eight) that can be used to store and transmit things like let-

ters and punctuation marks, we can start to build up to bigger and

better things. First, it is very easy to see that we can put bytes to-

10

MEANING

2ND DIGIT

1ST DIGIT

No

0

0

Maybe

0

1

Probably

1

0

Deﬁnitely

1

1

gether into lists in order to make a "string" of letters, what is often

referred to as a "character string." If we have a piece of text, like

"this is a piece of text" we can use a collection of bytes to represent

it like this:

011101000110100001101001011100110010000001101001011100110010

000001100001001000000111000001101001011001010110001101100101

001000000110111101100110001000000111010001100101011110000111

0100

Now nobody wants to look at that, let alone encode or decode it by

hand, but fortunately, the computers and software we use these

days takes care of the conversion and storage automatically. For ex-

ample, when we tell the open source data language "R" to store

"this is a piece of text" for us like this:

myText <- "this is a piece of text"

...we can be certain that inside the computer there is a long list of

zeroes and ones that represent the text that we just stored. By the

way, in order to be able to get our piece of text back later on, we

have made a kind of storage label for it (the word "myText" above).

Anytime that we want to remember our piece of text or use it for

something else, we can use the label "myText" to open up the

chunk of computer memory where we have put that long list of bi-

nary digits that represent our text. The left-pointing arrow made

up out of the less-than character ("<") and the dash character ("-")

gives R the command to take what is on the right hand side (the

quoted text) and put it into what is on the left hand side (the stor-

age area we have labeled "myText"). Some people call this the as-

signment arrow and it is used in some computer languages to

make it clear to the human who writes or reads it which direction

the information is ﬂowing.

From the computer’s standpoint, it is even simpler to store, remem-

ber, and manipulate numbers instead of text. Remember that an

eight bit byte can hold 256 combinations, so just using that very

small amount we could store the numbers from 0 to 255. (Of

course, we could have also done 1 to 256, but much of the counting

and numbering that goes on in computers starts with zero instead

of one.) Really, though, 255 is not much to work with. We couldn’t

count the number of houses in most towns or the number of cars in

a large parking garage unless we can count higher than 255. If we

put together two bytes to make 16 bits we can count from zero up

to 65,535, but that is still not enough for some of the really big num-

bers in the world today (for example, there are more than 200 mil-

lion cars in the U.S. alone). Most of the time, if we want to be ﬂexi-

ble in representing an integer (a number with no decimals), we use

four bytes stuck together. Four bytes stuck together is a total of 32

bits, and that allows us to store an integer as high as 4,294,967,295.

Things get slightly more complicated when we want to store a

negative number or a number that has digits after the decimal

point. If you are curious, try looking up "two's complement" for

more information about how signed numbers are stored and "ﬂoat-

ing point" for information about how numbers with digits after the

decimal point are stored. For our purposes in this book, the most

important thing to remember is that text is stored differently than

numbers, and among numbers integers are stored differently than

ﬂoating point. Later we will ﬁnd that it is sometimes necessary to

convert between these different representations, so it is always im-

portant to know how it is represented.

11

So far we have mainly looked at how to store one thing at a time,

like one number or one letter, but when we are solving problems

with data we often need to store a group of related things together.

The simplest place to start is with a list of things that are all stored

in the same way. For example, we could have a list of integers,

where each thing in the list is the age of a person in your family.

The list might look like this: 43, 42, 12, 8, 5. The ﬁrst two numbers

are the ages of the parents and the last three numbers are the ages

of the kids. Naturally, inside the computer each number is stored

in binary, but fortunately we don’t have to type them in that way

or look at them that way. Because there are no decimal points,

these are just plain integers and a 32 bit integer (4 bytes) is more

than enough to store each one. This list contains items that are all

the same "type" or "mode." The open source data program "R" re-

fers to a list where all of the items are of the same mode as a "vec-

tor." We can create a vector with R very easily by listing the num-

bers, separated by commas and inside parentheses:

c(43, 42, 12, 8, 5)

The letter "c" in front of the opening parenthesis stands for concate-

nate, which means to join things together. Slightly obscure, but

easy enough to get used to with some practice. We can also put in

some of what we learned a above to store our vector in a named lo-

cation (remember that a vector is list of items of the same mode/

type):

myFamilyAges <- c(43, 42, 12, 8, 5)

We have just created our ﬁrst "data set." It is very small, for sure,

only ﬁve items, but also very useful for illustrating several major

concepts about data. Here’s a recap:

•

In the heart of the computer, all data are represented in binary.

One binary digit, or bit, is the smallest chunk of data that we can

send from one place to another.

•

Although all data are at heart binary, computers and software

help to represent data in more convenient forms for people to

see. Three important representations are: "character" for repre-

senting text, "integer" for representing numbers with no digits

after the decimal point, and "ﬂoating point" for numbers that

may have digits after the decimal point. The list of numbers in

our tiny data set just above are integers.

•

Numbers and text can be collected into lists, which the open

source program "R" calls vectors. A vector has a length, which is

the number of items in it, and a "mode" which is the type of data

stored in the vector. The vector we were just working on has a

length of 5 and a mode of integer.

•

In order to be able to remember where we stored a piece of data,

most computer programs, including R, give us a way of labeling

a chunk of computer memory. We chose to give the 5-item vector

up above the name "myFamilyAges." Some people might refer to

this named list as a "variable," because the value of it varies, de-

pending upon which member of the list you are examining.

•

If we gather together one or more variables into a sensible

group, we can refer to them together as a "data set." Usually, it

doesn’t make sense to refer to something with just one variable

as a data set, so usually we need at least two variables. Techni-

cally, though, even our very simple "myFamilyAges" counts as a

data set, albeit a very tiny one.

12

Later in the book we will install and run the open source "R" data

program and learn more about how to create data sets, summarize

the information in those data sets, and perform some simple calcu-

lations and transformations on those data sets.

Chapter Challenge

Discover the meaning of "Boolean Logic" and the rules for "and",

"or", "not", and "exclusive or". Once you have studied this for a

while, write down on a piece of paper, without looking, all of the

binary operations that demonstrate these rules.

Sources

http://en.wikipedia.org/wiki/Claude_Shannon

http://en.wikipedia.org/wiki/Information_theory

http://cran.r-project.org/doc/manuals/R-intro.pdf

http://www.khanacademy.org/math/vi-hart/v/binary-hand-dan

ce

http://www.khanacademy.org/science/computer-science/v/intr

oduction-to-programs-data-types-and-variables

http://www.asciitable.com/

Test Yourself

13

Review 1.1 About Data

Check Answer

Question 1 of 3

The smallest unit of information com-

monly in use in today’s computers is

called:

A.

A Bit

B.

A Byte

C. A Nybble

D. An Integer

Data Science is different from other areas such as mathematics or statistics. Data Science is an applied

activity and data scientists serve the needs and solve the problems of data users. Before you can solve

a problem, you need to identify it and this process is not always as obvious as it might seem. In this

chapter, we discuss the identiﬁcation of data problems.

CHAPTER 2

14

Identifying Data Problems

Apple farmers live in constant fear, ﬁrst for their blossoms and

later for their fruit. A late spring frost can kill the blossoms. Hail or

extreme wind in the summer can damage the fruit. More generally,

farming is an activity that is ﬁrst and foremost in the physical

world, with complex natural processes and forces, like weather,

that are beyond the control of humankind.

In this highly physical world of unpredictable natural forces, is

there any role for data science? On the surface there does not seem

to be. But how can we know for sure? Having a nose for identify-

ing data problems requires openness, curiosity, creativity, and a

willingness to ask a lot of questions. In fact, if you took away from

the ﬁrst chapter the impression that a data scientist sits in front a of

computer all day and works a crazy program like R, that is a mis-

take. Every data scientist must (eventually) become immersed in

the problem domain where she is working. The data scientist may

never actually become a farmer, but if you are going to identify a

data problem that a farmer has, you have to learn to think like a

farmer, to some degree.

To get this domain knowledge you can read or watch videos, but

the best way is to ask "subject matter experts" (in this case farmers)

about what they do. The whole process of asking questions de-

serves its own treatment, but for now there are three things to

think about when asking questions. First, you want the subject mat-

ter experts, or SMEs, as they are sometimes called, to tell stories of

what they do. Then you want to ask them about anomalies: the un-

usual things that happen for better or for worse. Finally, you want

to ask about risks and uncertainty: what are the situations where it

is hard to tell what will happen next - and what happens next

could have a profound effect on whether the situation ends badly

or well. Each of these three areas of questioning reﬂects an ap-

proach to identifying data problems that may turn up something

good that could be accomplished with data, information, and the

right decision at the right time.

The purpose of asking about stories is that people mainly think in

stories. From farmers to teachers to managers to CEOs, people

know and tell stories about success and failure in their particular

domain. Stories are powerful ways of communicating wisdom be-

tween different members of the same profession and they are ways

of collecting a sense of identity that sets one profession apart from

another profession. The only problem is that stories can be wrong.

If you can get a professional to tell the main stories that guide how

she conducts her work, you can then consider how to verify those

stories. Without questioning the veracity of the person that tells the

story, you can imagine ways of measuring the different aspects of

how things happen in the story with an eye towards eventually

verifying (or sometimes debunking) the stories that guide profes-

sional work.

For example, the farmer might say that in the deep spring frost

that occurred ﬁve years ago, the trees in the hollow were spared

frost damage while the trees around the ridge of the hill had more

damage. For this reason, on a cold night the farmer places most of

the smudgepots (containers that hold a fuel that creates a smoky

ﬁre) around the ridge. The farmer strongly believes that this strat-

egy works, but does it? It would be possible to collect time-series

temperature data from multiple locations within the orchard on

cold and warm nights, and on nights with and without smudge-

pots. The data could be used to create a model of temperature

15

changes in the different areas of the orchard and this model could

support, improve, or debunk the story.

A second strategy for problem identiﬁcation is to look for the excep-

tion cases, both good and bad. A little later in the book we will

learn about how the core of classic methods of statistical inference

is to characterize "the center" - the most typical cases that occur -

and then examine the extreme cases that are far from the center for

information that could help us understand an intervention or an

unusual combination of circumstances. Identifying unusual cases

is a powerful way of understanding how things work, but it is nec-

essary ﬁrst to deﬁne the central or most typical occurrences in or-

der to have an accurate idea of what constitutes an unusual case.

Coming back to our farmer friend, in advance of a thunderstorm

late last summer, a powerful wind came through the orchard, tear-

ing the fruit off the trees. Most of the trees lost a small amount of

fruit: the dropped apples could be seen near the base of the tree.

One small grouping of trees seemed to lose a much larger amount

of fruit, however, and the drops were apparently scattered much

further from the trees. Is it possible that some strange wind condi-

tions made the situation worse in this one spot? Or is it just a mat-

ter of chance that a few trees in the same area all lost a bit more

fruit than would be typical.

A systematic count of lost fruit underneath a random sample of

trees would help to answer this question. The bulk of the trees

would probably have each lost about the same amount, but more

importantly, that "typical" group would give us a yardstick against

which we could determine what would really count as unusual.

When we found an unusual set of cases that was truly beyond the

limits of typical, we could rightly focus our attention on these to

try to understand the anomaly.

A third strategy for identifying data problems is to ﬁnd out about

risk and uncertainty. If you read the previous chapter you may re-

member that a basic function of information is to reduce uncer-

tainty. It is often valuable to reduce uncertainty because of how

risk affects the things we all do. At work, at school, at home, life is

full of risks: making a decision or failing to do so sets off a chain of

events that may lead to something good or something not so good.

It is difﬁcult to say, but in general we would like to narrow things

down in a way that maximizes the chances of a good outcome and

minimizes the chance of a bad one. To do this, we need to make bet-

ter decisions and to make better decisions we need to reduce uncer-

tainty. By asking questions about risks and uncertainty (and deci-

sions) a data scientist can zero in on the problems that matter. You

can even look at the previous two strategies - asking about the sto-

ries that comprise professional wisdom and asking about

anomalies/unusual cases - in terms of the potential for reducing

uncertainty and risk.

In the case of the farmer, much of the risk comes from the weather,

and the uncertainty revolves around which countermeasures will

be cost effective under prevailing conditions. Consuming lots of ex-

pensive oil in smudgepots on a night that turns out to be quite

warm is a waste of resources that could make the difference be-

tween a proﬁtable or an unproﬁtable year. So more precise and

timely information about local weather conditions might be a key

focus area for problem solving with data. What if a live stream of

national weather service doppler radar could appear on the

farmer’s smart phone? Let’s build an app for that...

16

"R" is an open source software program, developed by volunteers as a service to the community of

scientists, researchers, and data analysts who use it. R is free to download and use. Lots of advice and

guidance is available online to help users learn R, which is good because it is a powerful and complex

program, in reality a full featured programming language dedicated to data.

CHAPTER 3

17

Getting Started with R

If you are new to computers, programming, and/or data science

welcome to an exciting chapter that will open the door to the most

powerful free data analytics tool ever created anywhere in the uni-

verse, no joke. On the other hand, if you are experienced with

spreadsheets, statistical analysis, or accounting software you are

probably thinking that this book has now gone off the deep end,

never to return to sanity and all that is good and right in user inter-

face design. Both perspectives are reasonable. The "R" open source

data analysis program is immensely powerful, ﬂexible, and espe-

cially "extensible" (meaning that people can create new capabilities

for it quite easily). At the same time, R is "command line" oriented,

meaning that most of the work that one needs to perform is done

through carefully crafted text instructions, many of which have

tricky syntax (the punctuation and related rules for making a com-

mand that works). In addition, R is not especially good at giving

feedback or error messages that help the user to repair mistakes or

ﬁgure out what is wrong when results look funny.

But there is a method to the madness here. One of the virtues of R

as a teaching tool is that it hides very little. The successful user

must fully understand what the "data situation" is or else the R

commands will not work. With a spreadsheet, it is easy to type in a

lot of numbers and a formula like =FORECAST() and a result pops

into a cell like magic, whether it makes any sense or not. With R

you have to know your data, know what you can do with it, know

how it has to be transformed, and know how to check for prob-

lems. Because R is a programming language, it also forces users to

think about problems in terms of data objects, methods that can be

applied to those objects, and procedures for applying those meth-

ods. These are important metaphors used in modern programming

languages, and no data scientist can succeed without having at

least a rudimentary understanding of how software is pro-

grammed, tested, and integrated into working systems. The extensi-

bility of R means that new modules are being added all the time by

volunteers: R was among the ﬁrst analysis programs to integrate

capabilities for drawing data directly from the Twitter(r) social me-

dia platform. So you can be sure that whatever the next big devel-

opment is in the world of data, that someone in the R community

will start to develop a new "package" for R that will make use of it.

Finally, the lessons one learns in working with R are almost univer-

sally applicable to other programs and environments. If one has

mastered R, it is a relatively small step to get the hang of the SAS(r)

statistical programming language and an even smaller step to be-

ing able to follow SPSS(r) syntax. (SAS and SPSS are two of the

most widely used commercial statistical analysis programs). So

with no need for any licensing fees paid by school, student, or

teacher it is possible to learn the most powerful data analysis sys-

tem in the universe and take those lessons with you no matter

where you go. It will take a bit of patience though, so please hang

in there!

Let’s get started. Obviously you will need a computer. If you are

working on a tablet device or smartphone, you may want to skip

forward to the chapter on R-Studio, because regular old R has not

yet been reconﬁgured to work on tablet devices (but there is a

workaround for this that uses R-studio). There are a few experi-

ments with web-based interfaces to R, like this one -

http://dssm.unipa.it/R-php/R-php-1/R/ - but they are still in a

very early stage. If your computer has the Windows(r), Mac-OS-

X(r) or a Linux operating system, there is a version of R waiting for

you at http://cran.r-project.org/. Download and install your own

copy. If you sometimes have difﬁculties with installing new soft-

18

ware and you need some help, there is a wonderful little book by

Thomas P. Hogan called, Bare Bones R: A Brief Introductory Guide

that you might want to buy or borrow from your library. There are

lots of sites online that also give help with installing R, although

many of them are not oriented towards the inexperienced user. I

searched online using the term "help installing R" and I got a few

good hits. One site that was quite informative for installing R on

Windows was at "readthedocs.org," and you can try to access it at

this TinyUrl: http://tinyurl.com/872ngtt. For Mac users there is a

video by Jeremy Taylor at Vimeo.com,

http://vimeo.com/36697971, that outlines both the initial installa-

tion on a Mac and a number of other optional steps for getting

started. YouTube also had four videos that provide brief tutorials

for installing R. Try searching for "install R" in the YouTube search

box. The rest of this chapter assumes that you have installed R and

can run it on your computer as shown in the screenshot above.

(Note that this screenshot is from the Mac version of R: if you are

running Windows or Linux your R screen may appear slightly dif-

ferent from this.) Just for fun, one of the ﬁrst things you can do

when you have R running is to click on the color wheel and cus-

tomize the appearance of R. This screen shot uses Syracuse orange

as a background color. The screenshot also shows a simple com-

mand to type that shows the most basic method of interaction with

R. Notice near the bottom of the screenshot a greater than (">")

symbol. This is the command prompt: When R is running and it is

the active application on your desktop, if you type a command it

appears after the ">" symbol. If you press the "enter" or "return"

key, the command is sent to R for processing. When the processing

is done, a result may appear just under the ">." When R is done

processing, another command prompt (">") appears and R is ready

for your next command. In the screen shot, the user has typed

"1+1" and pressed the enter key. The formula 1+1 is used by ele-

mentary school students everywhere to insult each other’s math

skills, but R dutifully reports the result as 2. If you are a careful ob-

server, you will notice that just before the 2 there is a "1" in brack-

ets, like this: [1]. That [1] is a line number that helps to keep track

of the results that R displays. Pretty pointless when only showing

one line of results, but R likes to be consistent, so we will see quite

a lot of those numbers in brackets as we dig deeper.

19

Remember the list of ages of family members from the About Data

chapter? No? Well, here it is again: 43, 42, 12, 8, 5, for dad, mom,

sis, bro, and the dog, respectively. We mentioned that this was a list

of items, all of the same mode, namely "integer." Remember that

you can tell that they are OK to be integers because there are no

decimal points and therefore nothing after the decimal point. We

can create a vector of integers in r using the "c()" command. Take a

look at the screen shot just above.

This is just about the last time that the whole screenshot from the R

console will appear in the book. From here on out we will just look

at commands and output so we don’t waste so much space on the

page. The ﬁrst command line in the screen shot is exactly what ap-

peared in an earlier chapter:

c(43, 42, 12, 8, 5)

You may notice that on the following line, R dutifully reports the

vector that you just typed. After the line number "[1]", we see the

list 43, 42, 12, 8, and 5. R "echoes" this list back to us, because we

didn’t ask it to store the vector anywhere. In contrast, the next com-

mand line (also the same as in the previous chapter), says:

myFamilyAges <- c(43, 42, 12, 8, 5)

We have typed in the same list of numbers, but this time we have

assigned it, using the left pointing arrow, into a storage area that

we have named "myFamilyAges." This time, R responds just with

an empty command prompt. That’s why the third command line

requests a report of what myFamilyAges contains (Look after the

yellow ">". The text in blue is what you should type.) This is a sim-

ple but very important tool. Any time you want to know what is in

a data object in R, just type the name of the object and R will report

it back to you. In the next command we begin to see the power of

R:

sum(myFamilyAges)

This command asks R to add together all of the numbers in

myFamilyAges, which turns out to be 110 (you can check it your-

self with a calculator if you want). This is perhaps a bit of a weird

thing to do with the ages of family members, but it shows how

20

with a very short and simple command you can unleash quite a bit

of processing on your data. In the next line we ask for the "mean"

(what non-data people call the average) of all of the ages and this

turns out to be 22 years. The command right afterwards, called

"range," shows the lowest and highest ages in the list. Finally, just

for fun, we tried to issue the command "ﬁsh(myFamilyAges)."

Pretty much as you might expect, R does not contain a "ﬁsh()" func-

tion and so we received an error message to that effect. This shows

another important principle for working with R: You can freely try

things out at anytime without fear of breaking anything. If R can’t

understand what you want to accomplish, or you haven’t quite ﬁg-

ured out how to do something, R will calmly respond with an error

message and will not make any other changes until you give it a

new command. The error messages from R are not always super

helpful, but with some strategies that the book will discuss in fu-

ture chapters you can break down the problem and ﬁgure out how

to get R to do what you want.

Let’s take stock for a moment. First, you should deﬁnitely try all of

the commands noted above on your own computer. You can read

about the commands in this book all you want, but you will learn a

lot more if you actually try things out. Second, if you try a com-

mand that is shown in these pages and it does not work for some

reason, you should try to ﬁgure out why. Begin by checking your

spelling and punctuation, because R is very persnickety about how

commands are typed. Remember that capitalization matters in R:

myFamilyAges is not the same as myfamilyages. If you verify that

you have typed a command just as you see in the book and it still

does not work, try to go online and look for some help. There’s lots

of help at http://stackoverﬂow.com, at https://stat.ethz.ch, and

also at http://www.statmethods.net/. If you can ﬁgure out what

went wrong on your own you will probably learn something very

valuable about working with R. Third, you should take a moment

to experiment a bit with each new set of commands that you learn.

For example, just using the commands discussed earlier in the

chapter you could do this totally new thing:

myRange <- range(myFamilyAges)

What would happen if you did that command, and then typed

"myRange" (without the double quotes) on the next command line

to report back what is stored there ? What would you see? Then

think about how that worked and try to imagine some other experi-

ments that you could try. The more you experiment on your own,

the more you will learn. Some of the best stuff ever invented for

computers was the result of just experimenting to see what was

possible. At this point, with just the few commands that you have

already tried, you already know the following things about R (and

about data):

•

How to install R on your computer and run it.

•

How to type commands on the R console.

•

The use of the "c()" function. Remember that "c" stands for con-

catenate, which just means to join things together. You can put a

list of items inside the parentheses, separated by commas.

•

That a vector is pretty much the most basic form of data storage

in R, and that it consists of a list of items of the same mode.

•

That a vector can be stored in a named location using the assign-

ment arrow (a left pointing arrow made of a dash and a less than

symbol, like this: "<-").

21

•

That you can get a report of the data object that is in any named

location just by typing that name at the command line.

•

That you can "run" a function, such as mean(), on a vector of

numbers to transform them into something else. (The mean()

function calculates the average, which is one of the most basic

numeric summaries there is.)

•

That sum(), mean(), and range() are all legal functions in R

whereas ﬁsh() is not.

In the next chapter we will move forward a step or two by starting

to work with text and by combining our list of family ages with the

names of the family members and some other information about

them.

Chapter Challenge

Using logic and online resources to get help if you need it, learn

how to use the c() function to add another family member’s age on

the end of the myFamilyAges vector.

Sources

http://a-little-book-of-r-for-biomedical-statistics.readthedocs.org/

en/latest/src/installr.html

http://cran.r-project.org/

http://dssm.unipa.it/R-php/R-php-1/R/ (UNIPA experimental

web interface to R)

http://en.wikibooks.org/wiki/R_Programming

https://plus.google.com/u/0/104922476697914343874/posts (Jer-

emy Taylor’s blog: Stats Make Me Cry)

http://stackoverﬂow.com

https://stat.ethz.ch

http://www.statmethods.net/

22

Test Yourself

R Functions Used in This Chapter

c()! ! Concatenates data elements together

<- ! ! Assignment arrow

sum()! Adds data elements

range()! Min value and max value

mean()! The average

Review 3.1 Getting Started with R

Check Answer

Question 1 of 3

What is the cost of each software license for the R open

source data analysis program?

A.

R is free

B.

99 cents in the iTunes store

C. $10

D. $100

23

An old adage in detective work is to, "follow the money." In data science, one key to success is to

"follow the data." In most cases, a data scientist will not help to design an information system from

scratch. Instead, there will be several or many legacy systems where data resides; a big part of the

challenge to the data scientist lies in integrating those systems.

CHAPTER 4

24

Follow the Data

Hate to nag, but have you had a checkup lately? If you have been

to the doctor for any reason you may recall that the doctor’s ofﬁce

is awash with data. First off, the doctor has loads of digital sensors,

everything from blood pressure monitors to ultrasound machines,

and all of these produce mountains of data. Perhaps of greater con-

cern in this era of debate about health insurance, the doctors ofﬁce

is one of the big jumping off points for ﬁnancial and insurance

data. One of the notable "features" of the U.S. healthcare system is

our most common method of healthcare delivery: paying by the

procedure. When you experience a "procedure" at the doctor’s of-

ﬁce, whether it is a consultation, an examination, a test, or some-

thing else, this initiates a chain of data events with far reaching con-

sequences.

If your doctor is typical, the starting point of these events is a pa-

per form. Have you ever looked at one of these in detail? Most of

the form will be covered by a large matrix of procedures and

codes. Although some of the better equipped places may use this

form digitally on a tablet or other computer, paper forms are still

ubiquitous. Somewhere either in the doctor’s ofﬁce or at an out-

sourced service company, the data on the paper form are entered

into a system that begins the insurance reimbursement and/or bill-

ing process.

Where do these procedure data go? What other kinds of data (such

as patient account information) may get attached to them in a sub-

sequent step? What kinds of networks do these linked data travel

over, and what kind of security do they have? How many steps are

there in processing the data before they get to the insurance com-

pany? How does the insurance company process and analyze the

data before issuing the reimbursement? How is the money "trans-

mitted" once the insurance company’s systems have given ap-

proval to the reimbursement? These questions barely scratch the

surface: there are dozens or hundreds of processing steps that we

haven’t yet imagined.

It is easy to see from this example, that the likelihood of being able

to throw it all out and start designing a better or at least more stan-

dardized system from scratch is nil. But what if you had the job of

improving the efﬁciency of the system, or auditing the insurance

reimbursements to make sure they were compliant with insurance

records, or using the data to detect and predict outbreaks and epi-

demics, or providing feedback to consumers about how much they

can expect to pay out of pocket for various procedures?

The critical starting point for your project would be to follow the

data. You would need to be like a detective, ﬁnding out in a sub-

stantial degree of detail the content, format, senders, receivers,

transmission methods, repositories, and users of data at each step

in the process and at each organization where the data are proc-

essed or housed.

Fortunately there is an extensive area of study and practice called

"data modeling" that provides theories, strategies, and tools to help

with the data scientist’s goal of following the data. These ideas

started in earnest in the 1970s with the introduction by computer

scientist Ed Yourdon of a methodology called Data Flow Diagrams.

A more contemporary approach, that is strongly linked with the

practice of creating relational databases, is called the entity-

relationship model. Professionals using this model develop Entity-

Relationship Diagrams (ERDs) that describe the structure and

movement of data in a system.

25

Entity-relationship modeling occurs at different levels ranging

from an abstract conceptual level to a physical storage level. At the

conceptual level an entity is an object or thing, usually something

in the real world. In the doctor’s ofﬁce example, one important "ob-

ject" is the patient. Another entity is the doctor. The patient and the

doctor are linked by a relationship: in modern health care lingo

this is the "provider" relationship. If the patient is Mr. X and the

doctor is Dr. Y, the provider relationship provides a bidirectional

link:

•

Dr. Y is the provider for Mr. X

•

Mr. X’s provider is Dr. Y

Naturally there is a range of data that can represent Mr. X: name

address, age, etc. Likewise, there are data that represent Dr. Y:

years of experience as a doctor, specialty areas, certiﬁcations, li-

censes. Importantly, there is also a chunk of data that represents

the linkage between X and Y, and this is the relationship.

Creating an ERD requires investigating and enumerating all of the

entities, such as patients and doctors, as well as all of the relation-

ships that may exist among them. As the beginning of the chapter

suggested, this may have to occur across multiple organizations

(e.g., the doctor’s ofﬁce and the insurance company) depending

upon the purpose of the information system that is being designed.

Eventually, the ERDs must become detailed enough that they can

serve as a speciﬁcation for the physical storage in a database.

In an application area like health care, there are so many choices

for different ways of designing the data that it requires some expe-

rience and possibly some "art" to create a workable system. Part of

the art lies in understanding the users’ current information needs

and anticipating how those needs may change in the future. If an

organization is redesigning a system, adding to a system, or creat-

ing brand new systems, they are doing so in the expectation of a

future beneﬁt. This beneﬁt may arise from greater efﬁciency, reduc-

tion of errors/inaccuracies, or the hope of providing a new product

or service with the enhanced information capabilities.

Whatever the goal, the data scientist has an important and difﬁcult

challenge of taking the methods of today - including paper forms

and manual data entry - and imagining the methods of tomorrow.

Follow the data!

In the next chapter we look at one of the most common and most

useful ways of organizing data, namely in a rectangular structure

that has rows and columns. This rectangular arrangement of data

appears in spreadsheets and databases that are used for a variety

of applications. Understanding how these rows and columns are

organized is critical to most tasks in data science.

Sources

http://en.wikipedia.org/wiki/Data_modeling

http://en.wikipedia.org/wiki/Entity-relationship_diagram

26

One of the most basic and widely used methods of representing data is to use rows and columns,

where each row is a case or instance and each column is a variable and attribute. Most spreadsheets

arrange their data in rows and columns, although spreadsheets don’t usually refer to these as cases or

variables. R represents rows and columns in an object called a data frame.

CHAPTER 5

27

Rows and Columns

Although we live in a three dimensional world, where a box of ce-

real has height, width, and depth, it is a sad fact of modern life that

pieces of paper, chalkboards, whiteboards, and computer screens

are still only two dimensional. As a result, most of the statisticians,

accountants, computer scientists, and engineers who work with

lots of numbers tend to organize them in rows and columns.

There’s really no good reason for this other than it makes it easy to

ﬁll a rectangular piece of paper with numbers. Rows and columns

can be organized any way that you want, but the most common

way is to have the rows be "cases" or "instances" and the columns

be "attributes" or "variables." Take a look at this nice, two dimen-

sional representation of rows and columns:

Pretty obvious what’s going on, right? The top line, in bold, is not

really part of the data. Instead, the top line contains the attribute or

variable names. Note that computer scientists tend to call them at-

tributes while statisticians call them variables. Either term is OK.

For example, age is an attribute that every living thing has, and

you could count it in minutes, hours, days, months, years, or other

units of time. Here we have the Age attribute calibrated in years.

Technically speaking, the variable names in the top line are "meta-

data" or what you could think of as data about data. Imagine how

much more difﬁcult it would be to understand what was going on

in that table without the metadata. There’s lot of different kinds of

metadata: variable names are just one simple type of metadata.

So if you ignore the top row, which contains the variable names,

each of the remaining rows is an instance or a case. Again, com-

puter scientists may call them instances, and statisticians may call

them cases, but either term is ﬁne. The important thing is that each

row refers to an actual thing. In this case all of our things are living

creatures in a family. You could think of the Name column as "case

labels" in that each one of these labels refers to one and only one

row in our data. Most of the time when you are working with a

large dataset, there is a number used for the case label, and that

number is unique for each case (in other words, the same number

would never appear in more than one row). Computer scientists

sometimes refer to this column of unique numbers as a "key." A key

is very useful particularly for matching things up from different

data sources, and we will run into this idea again a bit later. For

now, though, just take note that the "Dad" row can be distin-

guished from the "Bro" row, even though they are both Male. Even

if we added an "Uncle" row that had the same Age, Gender, and

Weight as "Dad" we would still be able to tell the two rows apart

because one would have the name "Dad" and the other would have

the name "Uncle."

One other important note: Look how each column contains the

same kind of data all the way down. For example, the Age column

is all numbers. There’s nothing in the Age column like "Old" or

"Young." This is a really valuable way of keeping things organized.

After all, we could not run the mean() function on the Age column

28

NAME

AGE

GENDER

WEIGHT

Dad

43

Male

188

Mom

42

Female

136

Sis

12

Female

83

Bro

8

Male

61

Dog

5

Female

44

if it contained a little piece of text, like "Old" or "Young." On a re-

lated note, every cell (that is an intersection of a row and a column,

for example, Sis’s Age) contains just one piece of information. Al-

though a spreadsheet or a word processing program might allow

us to put more than one thing in a cell, a real data handling pro-

gram will not. Finally, see that every column has the same number

of entries, so that the whole forms a nice rectangle. When statisti-

cians and other people who work with databases work with a data-

set, they expect this rectangular arrangement.

Now let’s ﬁgure out how to get these rows and columns into R.

One thing you will quickly learn about R is that there is almost al-

ways more than one way to accomplish a goal. Sometimes the

quickest or most efﬁcient way is not the easiest to understand. In

this case we will build each column one by one and then join them

together into a single data frame. This is a bit labor intensive, and

not the usual way that we would work with a data set, but it is

easy to understand. First, run this command to make the column

of names:

myFamilyNames <- c("Dad","Mom","Sis","Bro","Dog")

One thing you might notice is that every name is placed within

double quotes. This is how you signal to R that you want it to treat

something as a string of characters rather than the name of a stor-

age location. If we had asked R to use Dad instead of "Dad" it

would have looked for a storage location (a data object) named

Dad. Another thing to notice is that the commas separating the dif-

ferent values are outside of the double quotes. If you were writing

a regular sentence this is not how things would look, but for com-

puter programming the comma can only do its job of separating

the different values if it is not included inside the quotes. Once you

have typed the line above, remember that you can check the con-

tents of myFamilyNames by typing it on the next command line:

myFamilyNames

The output should look like this:

[1] "Dad" "Mom" "Sis" "Bro" "Dog"

Next, you can create a vector of the ages of the family members,

like this:

myFamilyAges <- c(43, 42, 12, 8, 5)

Note that this is exactly the same command we used in the last

chapter, so if you have kept R running between then and now you

would not even have to retype this command because

myFamilyAges would still be there. Actually, if you closed R since

working the examples from the last chapter you will have been

prompted to "save the workspace" and if you did so, then R re-

stored all of the data objects you were using in the last session. You

can always check by typing myFamilyAges on a blank command

line. The output should look like this:

[1] 43 42 12 8 5

Hey, now you have used the c() function and the assignment arrow

to make myFamilyNames and myFamilyAges. If you look at the

data table earlier in the chapter you should be able to ﬁgure out the

commands for creating myFamilyGenders and myFamilyWeights.

In case you run into trouble, these commands also appear on the

next page, but you should try to ﬁgure them out for yourself before

you turn the page. In each case after you type the command to cre-

ate the new data object, you should also type the name of the data

29

object at the command line to make sure that it looks the way it

should. Four variables, each one with ﬁve values in it. Two of the

variables are character data and two of the variables are integer

data. Here are those two extra commands in case you need them:

myFamilyGenders <- c("Male","Female","Female","Male","Female")

myFamilyWeights <- c(188,136,83,61,44)

Now we are ready to tackle the dataframe. In R, a dataframe is a

list (of columns), where each element in the list is a vector. Each

vector is the same length, which is how we get our nice rectangular

row and column setup, and generally each vector also has its own

name. The command to make a data frame is very simple:

myFamily <- data.frame(myFamilyNames, +

myFamilyAges, myFamilyGenders, myFamilyWeights)

Look out! We’re starting to get commands that are long enough

that they break onto more than one line. The + at the end of the

ﬁrst line tells R to wait for more input on the next line before trying

to process the command. If you want to, you can type the whole

thing as one line in R, but if you do, just leave out the plus sign.

Anyway, the data.frame() function makes a dataframe from the

four vectors that we previously typed in. Notice that we have also

used the assignment arrow to make a new stored location where R

puts the data frame. This new data object, called myFamily, is our

dataframe. Once you have gotten that command to work, type

myFamily at the command line to get a report back of what the

data frame contains. Here’s the output you should see:

myFamilyNames myFamilyAges myFamilyGenders myFamilyWeights

1 Dad 43 Male 188

2 Mom 42 Female 136

3 Sis 12 Female 83

4 Bro 8 Male 61

5 Dog 5 Female 44

This looks great. Notice that R has put row numbers in front of

each row of our data. These are different from the output line num-

bers we saw in brackets before, because these are actual "indices"

into the data frame. In other words, they are the row numbers that

R uses to keep track of which row a particular piece of data is in.

With a small data set like this one, only ﬁve rows, it is pretty easy

just to take a look at all of the data. But when we get to a bigger

data set this won’t be practical. We need to have other ways of sum-

marizing what we have. The ﬁrst method reveals the type of "struc-

ture" that R has used to store a data object.

> str(myFamily)

'data.frame':! 5 obs. of 4 variables:

$ myFamilyNames : Factor w/ 5 levels

!!"Bro","Dad","Dog",..: 2 4 5 1 3

$ myFamilyAges : num 43 42 12 8 5

$ myFamilyGenders: Factor w/ 2 levels

!!"Female","Male": 2 1 1 2 1

$ myFamilyWeights: num 188 136 83 61 44

Take note that for the ﬁrst time, the example shows the command

prompt ">" in order to differentiate the command from the output

that follows. You don’t need to type this: R provides it whenever it

is ready to receive new input. From now on in the book, there will

30

be examples of R commands and output that are mixed together,

so always be on the lookout for ">" because the command after

that is what you have to type.

OK, so the function "str()" reveals the structure of the data object

that you name between the parentheses. In this case we pretty well

knew that myFamily was a data frame because we just set that up

in a previous command. In the future, however, we will run into

many situations where we are not sure how R has created a data

object, so it is important to know str() so that you can ask R to re-

port what an object is at any time.

In the ﬁrst line of output we have the conﬁrmation that myFamily

is a data frame as well as an indication that there are ﬁve observa-

tions ("obs." which is another word that statisticians use instead of

cases or instances) and four variables. After that ﬁrst line of output,

we have four sections that each begin with "$". For each of the four

variables, these sections describe the component columns of the

myFamily dataframe object.

Each of the four variables has a "mode" or type that is reported by

R right after the colon on the line that names the variable:

$ myFamilyGenders: Factor w/ 2 levels

For example, myFamilyGenders is shown as a "Factor." In the termi-

nology that R uses, Factor refers to a special type of label that can

be used to identify and organize groups of cases. R has organized

these labels alphabetically and then listed out the ﬁrst few cases

(because our dataframe is so small it actually is showing us all of

the cases). For myFamilyGenders we see that there are two "lev-

els," meaning that there are two different options: female and male.

R assigns a number, starting with one, to each of these levels, so

every case that is "Female" gets assigned a 1 and every case that is

"Male" gets assigned a 2 (because Female comes before Male in the

alphabet, so Female is the ﬁrst Factor label, so it gets a 1). If you

have your thinking cap on, you may be wondering why we started

out by typing in small strings of text, like "Male," but then R has

gone ahead and converted these small pieces of text into numbers

that it calls "Factors." The reason for this lies in the statistical ori-

gins of R. For years, researchers have done things like calling an ex-

perimental group "Exp" and a control, group "Ctl" without intend-

ing to use these small strings of text for anything other than labels.

So R assumes, unless you tell it otherwise, that when you type in a

short string like "Male" that you are referring to the label of a

group, and that R should prepare for the use of Male as a "Level" of

a "Factor." When you don’t want this to happen you can instruct R

to stop doing this with an option on the data.frame() function:

stringsAsFactors=FALSE. We will look with more detail at options

and defaults a little later on.

Phew, that was complicated! By contrast, our two numeric vari-

ables, myFamilyAges and myFamilyWeights, are very simple. You

can see that after the colon the mode is shown as "num" (which

stands for numeric) and that the ﬁrst few values are reported:

$ myFamilyAges : num 43 42 12 8 5

Putting it all together, we have pretty complete information about

the myFamily dataframe and we are just about ready to do some

more work with it. We have seen ﬁrsthand that R has some pretty

cryptic labels for things as well as some obscure strategies for con-

verting this to that. R was designed for experts, rather than nov-

31

ices, so we will just have to take our lumps so that one day we can

be experts too.

Next, we will examine another very useful function called sum-

mary(). Summary() provides some overlapping information to str()

but also goes a little bit further, particularly with numeric vari-

ables. Here’s what we get:

> summary(myFamily)

myFamilyNames myFamilyAges

Bro: 1!! ! Min.! : 5

Dad: 1!! ! 1st Qu.!: 8

Dog: 1!! ! Median! : 12

Mom: 1!! ! Mean! : 22!!!

Sis: 1 3rd Qu.!: 42

myFamilyGenders myFamilyWeights

Female : 3!! Min.! : 44

Male! : 2 ! 1st Qu. : 61.0

!!!!Median! : 83.0

!!!!Mean! : 102.4

!!!!3rd Qu.!: 136.0

!!!!Max!! : 188.0!!

In order to ﬁt on the page properly, these columns have been reor-

ganized a bit. The name of a column/variable, sits up above the in-

formation that pertains to it, and each block of information is inde-

pendent of the others (so it is meaningless, for instance, that "Bro:

1" and "Min." happen to be on the same line of output). Notice, as

with str(), that the output is quite different depending upon

whether we are talking about a Factor, like myFamilyNames or

myFamilyGenders, versus a numeric variable like myFamilyAges

and myFamilyWeights. The columns for the Factors list out a few

of the Factor names along with the number of occurrences of cases

that are coded with that factor. So for instance, under

myFamilyGenders it shows three females and two males. In con-

trast, for the numeric variables we get ﬁve different calculated

quantities that help to summarize the variable. There’s no time like

the present to start to learn about what these are, so here goes:

•

"Min." refers to the minimum or lowest value among all the

cases. For this dataframe, 5 is the age of the dog and it is the low-

est age of all of the family members.

•

"1st Qu." refers to the dividing line at the top of the ﬁrst quartile.

If we took all the cases and lined them up side by side in order

of age (or weight) we could then divide up the whole into four

groups, where each group had the same number of observations.

32

1ST

QUARTILE

2ND

QUARTILE

3RD

QUARTILE

4TH

QUARTILE

25% of cases

with the

smallest

values here

25% of cases

just below

the median

here

25% of cases

just above

the mean

here

25% of cases

with the

largest

values here

Just like a number line, the smallest cases would be on the left

with the largest on the right. If we’re looking at myFamilyAges,

the leftmost group, which contains one quarter of all the cases,

would start with ﬁve on the low end (the dog) and would have

eight on the high end (Bro). So the "ﬁrst quartile" is the value of

age (or another variable) that divides the ﬁrst quarter of the

cases from the other three quarters. Note that if we don’t have a

number of cases that divides evenly by four, the value is an ap-

proximation.

•

Median refers to the value of the case that splits the whole group

in half, with half of the cases having higher values and half hav-

ing lower values. If you think about it a little bit, the median is

also the dividing line that separates the second quartile from the

third quartile.

•

Mean, as we have learned before, is the numeric average of all of

the values. For instance, the average age in the family is reported

as 22.

•

"3rd Qu." is the third quartile. If you remember back to the ﬁrst

quartile and the median, this is the third and ﬁnal dividing line

that splits up all of the cases into four equal sized parts. You may

be wondering about these quartiles and what they are useful for.

Statisticians like them because they give a quick sense of the

shape of the distribution. Everyone has the experience of sorting

and dividing things up - pieces of pizza, playing cards into

hands, a bunch of players into teams - and it is easy for most peo-

ple to visualize four equal sized groups and useful to know how

high you need to go in age or weight (or another variable) to get

to the next dividing line between the groups.

•

Finally, "Max" is the maximum value and as you might expect

displays the highest value among all of the available cases. For

example, in this dataframe Dad has the highest weight: 188.

Seems like a pretty trim guy.

Just one more topic to pack in before ending this chapter: How to

access the stored variables in our new dataframe. R stores the data-

frame as a list of vectors and we can use the name of the dataframe

together with the name of a vector to refer to each one using the "$"

to connect the two labels like this:

> myFamily$myFamilyAges

[1] 43 42 12 8 5

If you’re alert you might wonder why we went to the trouble of

typing out that big long thing with the $ in the middle, when we

could have just referred to "myFamilyAges" as we did earlier when

we were setting up the data. Well, this is a very important point.

When we created the myFamily dataframe, we copied all of the in-

formation from the individual vectors that we had before into a

brand new storage space. So now that we have created the my-

Family dataframe, myFamily$myFamilyAges actually refers to a

completely separate (but so far identical) vector of values. You can

prove this to yourself very easily, and you should, by adding some

data to the original vector, myFamilyAges:

> myFamilyAges <- c(myFamilyAges, 11)

> myFamilyAges

[1] 43 42 12 8 5 11

> myFamily$myFamilyAges

33

[1] 43 42 12 8 5

Look very closely at the ﬁve lines above. In the ﬁrst line, we use

the c() command to add the value 11 to the original list of ages that

we had stored in myFamilyAges (perhaps we have adopted an

older cat into the family). In the second line we ask R to report

what the vector myFamilyAges now contains. Dutifully, on the

third line above, R reports that myFamilyAges now contains the

original ﬁve values and the new value of 11 on the end of the list.

When we ask R to report myFamily$myFamilyAges, however, we

still have the original list of ﬁve values only. This shows that the da-

taframe and its component columns/vectors is now a completely

independent piece of data. We must be very careful, if we estab-

lished a dataframe that we want to use for subsequent analysis,

that we don’t make a mistake and keep using some of the original

data from which we assembled the dataframe.

Here’s a puzzle that follows on from this question. We have a nice

dataframe with ﬁve observations and four variables. This is a rec-

tangular shaped data set, as we discussed at the beginning of the

chapter. What if we tried to add on a new piece of data on the end

of one of the variables? In other words, what if we tried something

like this command:

myFamily$myFamilyAges<-c(myFamily$myFamilyAges, 11)

If this worked, we would have a pretty weird situation: The vari-

able in the dataframe that contained the family members’ ages

would all of a sudden have one more observation than the other

variables: no more perfect rectangle! Try it out and see what hap-

pens. The result helps to illuminate how R approaches situations

like this.

So what new skills and knowledge do we have at this point? Here

are a few of the key points from this chapter:

•

In R, as in other programs, a vector is a list of elements/things

that are all of the same kind, or what R refers to as a mode. For

example, a vector of mode "numeric" would contain only num-

bers.

•

Statisticians, database experts and others like to work with rec-

tangular datasets where the rows are cases or instances and the

columns are variables or attributes.

•

In R, one of the typical ways of storing these rectangular struc-

tures is in an object known as a dataframe. Technically speaking

a dataframe is a list of vectors where each vector has the exact

same number of elements as the others (making a nice rectan-

gle).

•

In R, the data.frame() function organizes a set of vectors into a

dataframe. A dataframe is a conventional, rectangular shaped

data object where each column is a vector of uniform mode and

having the same number of elements as the other columns in the

dataframe. Data are copied from the original source vectors into

new storage space. The variables/columns of the dataframe can

be accessed using "$" to connect the name of the dataframe to the

name of the variable/column.

•

The str() and summary() functions can be used to reveal the

structure and contents of a dataframe (as well as of other data ob-

jects stored by R). The str() function shows the structure of a data

object, while summary() provides numerical summaries of nu-

meric variables and overviews of non-numeric variables.

34

•

A factor is a labeling system often used to organize groups of

cases or observations. In R, as well as in many other software

programs, a factor is represented internally with a numeric ID

number, but factors also typically have labels like "Male" and

"Female" or "Experiment" and "Control." Factors always have

"levels," and these are the different groups that the factor signi-

ﬁes. For example, if a factor variable called Gender codes all

cases as either "Male" or "Female" then that factor has exactly

two levels.

•

Quartiles are a division of a sorted vector into four evenly sized

groups. The ﬁrst quartile contains the lowest-valued elements,

for example the lightest weights, whereas the fourth quartile con-

tains the highest-valued items. Because there are four groups,

there are three dividing lines that separate them. The middle di-

viding line that splits the vector exactly in half is the median.

The term "ﬁrst quartile" often refers to the dividing line to the

left of the median that splits up the lower two quarters and the

value of the ﬁrst quartile is the value of the element of the vector

that sits right at that dividing line. Third quartile is the same

idea, but to the right of the median and splitting up the two

higher quarters.

•

Min and max are often used as abbreviations for minimum and

maximum and these are the terms used for the highest and low-

est values in a vector. Bonus: The "range" of a set of numbers is

the maximum minus the minimum.

•

The mean is the same thing that most people think of as the aver-

age. Bonus: The mean and the median are both measures of

what statisticians call "central tendency."

Chapter Challenge

Create another variable containing information about family mem-

bers (for example, each family member’s estimated IQ; you can

make up the data). Take that new variable and put it in the existing

35

Review 5.1 Rows and columns

Check Answer

Question 1 of 7

What is the name of the data object that R uses to store a rec-

tangular dataset of cases and variables?

A.

A list

B. A mode

C. A vector

D. A dataframe

myFamily dataframe. Rerun the summary() function on myFamily

to get descriptive information on your new variable.

Sources

http://en.wikipedia.org/wiki/Central_tendency

http://en.wikipedia.org/wiki/Median

http://en.wikipedia.org/wiki/Relational_model

http://msenux.redwoods.edu/math/R/dataframe.php

http://stat.ethz.ch/R-manual/R-devel/library/base/html/data.fr

ame.html

http://www.burns-stat.com/pages/Tutor/hints_R_begin.html

http://www.khanacademy.org/math/statistics/v/mean-median-

and-mode

R Functions Used in This Chapter

c()! ! ! Concatenates data elements together

<-! ! ! Assignment arrow

data.frame()! Makes a dataframe from separate vectors

str()! ! ! Reports the structure of a data object

summary()! Reports data modes/types and a data overview

36

Many of the simplest and most practical methods for summarizing collections of numbers come to us

from four guys who were born in the 1800s at the start of the industrial revolution. A considerable

amount of the work they did was focused on solving real world problems in manufacturing and

agriculture by using data to describe and draw inferences from what they observed.

CHAPTER 6

37

Beer, Farms, and Peas

The end of the 1800s and the early 1900s were a time of astonishing

progress in mathematics and science. Given enough time, paper,

and pencils, scientists and mathematicians of that age imagined

that just about any problem facing humankind - including the limi-

tations of people themselves - could be measured, broken down,

analyzed, and rebuilt to become more efﬁcient. Four Englishmen

who epitomized both this scientiﬁc progress and these idealistic be-

liefs were Francis Galton, Karl Pearson, William Sealy Gosset, and

Ronald Fisher.

First on the scene was Francis Galton, a half-cousin to the more

widely known Charles Darwin, but quite the intellectual force him-

self. Galton was an English gentleman of independent means who

studied Latin, Greek, medicine, and mathematics, and who made a

name for himself as an African explorer. He is most widely known

as a proponent of "eugenics" and is credited with coining the term.

Eugenics is the idea that the human race could be improved

through selective breeding. Galton studied heredity in peas, rab-

bits, and people and concluded that certain people should be paid

to get married and have children because their offspring would im-

prove the human race. These ideas were later horribly misused in

the 20th century, most notably by the Nazis as a justiﬁcation for kill-

ing people because they belonged to supposedly inferior races. Set-

ting eugenics aside, however, Galton made several notable and

valuable contributions to mathematics and statistics, in particular

illuminating two basic techniques that are widely used today: corre-

lation and regression.

For all his studying and theorizing, Galton was not an outstanding

mathematician, but he had a junior partner, Karl Pearson, who is

often credited with founding the ﬁeld of mathematical statistics.

Pearson reﬁned the math behind correlation and regression and

did a lot else besides to contribute to our modern abilities to man-

age numbers. Like Galton, Pearson was a proponent of eugenics,

but he also is credited with inspiring some of Einstein’s thoughts

about relativity and was an early advocate of women’s rights.

Next to the statistical party was William Sealy Gosset, a wizard at

both math and chemistry. It was probably the latter expertise that

led the Guinness Brewery in Dublin Ireland to hire Gosset after col-

lege. As a forward looking business, the Guinness brewery was on

the lookout for ways of making batches of beer more consistent in

quality. Gosset stepped in and developed what we now refer to as

small sample statistical techniques - ways of generalizing from the

results of a relatively few observations. Of course, brewing a batch

of beer is a time consuming and expensive process, so in order to

draw conclusions from experimental methods applied to just a few

batches, Gosset had to ﬁgure out the role of chance in determining

how a batch of beer had turned out. Guinness frowned upon aca-

demic publications, so Gosset had to publish his results under the

modest pseudonym, "Student." If you ever hear someone discuss-

ing the "Student’s t-Test," that is where the name came from.

Last but not least among the born-in-the-1800s bunch was Ronald

Fisher, another mathematician who also studied the natural sci-

ences, in his case biology and genetics. Unlike Galton, Fisher was

not a gentleman of independent means, in fact, during his early

married life he and his wife struggled as subsistence farmers. One

of Fisher’s professional postings was to an agricultural research

farm called Rothhamsted Experimental Station. Here, he had ac-

cess to data about variations in crop yield that led to his develop-

ment of an essential statistical technique known as the analysis of

38

variance. Fisher also pioneered the area of experimental design,

which includes matters of factors, levels, experimental groups, and

control groups that we noted in the previous chapter.

Of course, these four are certainly not the only 19th and 20th cen-

tury mathematicians to have made substantial contributions to

practical statistics, but they are notable with respect to the applica-

tions of mathematics and statistics to the other sciences (and "Beer,

Farms, and Peas" makes a good chapter title as well).

One of the critical distinctions woven throughout the work of these

four is between the "sample" of data that you have available to ana-

lyze and the larger "population" of possible cases that may or do

exist. When Gosset ran batches of beer at the brewery, he knew

that it was impractical to run every possible batch of beer with

every possible variation in recipe and preparation. Gosset knew

that he had to run a few batches, describe what he had found and

then generalize or infer what might happen in future batches. This

is a fundamental aspect of working with all types and amounts of

data: Whatever data you have, there’s always more out there.

There’s data that you might have collected by changing the way

things are done or the way things are measured. There’s future

data that hasn’t been collected yet and might never be collected.

There’s even data that we might have gotten using the exact same

strategies we did use, but that would have come out subtly differ-

ent just due to randomness. Whatever data you have, it is just a

snapshot or "sample" of what might be out there. This leads us to

the conclusion that we can never, ever 100% trust the data we have.

We must always hold back and keep in mind that there is always

uncertainty in data. A lot of the power and goodness in statistics

comes from the capabilities that people like Fisher developed to

help us characterize and quantify that uncertainty and for us to

know when to guard against putting too much stock in what a sam-

ple of data have to say. So remember that while we can always de-

scribe the sample of data we have, the real trick is to infer what

the data may mean when generalized to the larger population of

data that we don’t have. This is the key distinction between de-

scriptive and inferential statistics.

We have already encountered several descriptive statistics in previ-

ous chapters, but for the sake of practice here they are again, this

time with the more detailed deﬁnitions:

•

The mean (technically the arithmetic mean), a measure of central

tendency that is calculated by adding together all of the observa-

tions and dividing by the number of observations.

•

The median, another measure of central tendency, but one that

cannot be directly calculated. Instead, you make a sorted list of

all of the observations in the sample, then go halfway up that

list. Whatever the value of the observation is at the halfway

point, that is the median.

•

The range, which is a measure of "dispersion" - how spread out a

bunch of numbers in a sample are - calculated by subtracting the

lowest value from the highest value.

To this list we should add three more that you will run into in a va-

riety of situations:

•

The mode, another measure of central tendency. The mode is the

value that occurs most often in a sample of data. Like the me-

dian, the mode cannot be directly calculated. You just have to

39

count up how many of each number there are and then pick the

category that has the most.

•

The variance, a measure of dispersion. Like the range, the vari-

ance describes how spread out a sample of numbers is. Unlike

the range, though, which just uses two numbers to calculate dis-

persion, the variance is obtained from all of the numbers

through a simple calculation that compares each number to the

mean. If you remember the ages of the family members from the

previous chapter and the mean age of 22, you will be able to

make sense out of the following table:

This table shows the calculation of the variance, which begins by

obtaining the "deviations" from the mean and then "squares" them

(multiply each times itself) to take care of the negative deviations

(for example, -14 from the mean for Bro). We add up all of the

squared deviations and then divide by the number of observations

to get a kind of "average squared deviation." Note that it was not a

mistake to divide by 4 instead of 5 - the reasons for this will be-

come clear later in the book when we examine the concept of de-

grees of freedom. This result is the variance, a very useful mathe-

matical concept that appears all over the place in statistics. While it

is mathematically useful, it is not too nice too look at. For instance,

in this example we are looking at the 356.5 squared-years of devia-

tion from the mean. Who measures anything in squared years?

Squared feet maybe, but that’s a different discussion. So, to address

this weirdness, statisticians have also provided us with:

•

The standard deviation, another measure of dispersion, and a

cousin to the variance. The standard deviation is simply the

square root of the variance, which puts us back in regular units

like "years." In the example above, the standard deviation would

be about 18.88 years (rounding to two decimal places, which is

plenty in this case).

Now let’s have R calculate some statistics for us:

> var(myFamily$myFamilyAges)

[1] 356.5

> sd(myFamily$myFamilyAges)

[1] 18.88121

Note that these commands carry on using the data used in the pre-

vious chapter, including the use of the $ to address variables

within a dataframe. If you do not have the data from the previous

chapter you can also do this:

> var(c(43,42,12,8,5))

[1] 356.5

40

WHO

AGE

AGE -

MEAN

(AGE-

MEAN)

2

Dad

43

43-22 = 21

21*21=441

Mom

42

42-22=20

20*20=400

Sis

12

12-22=-10

-10*-10=100

Bro

8

8-22=-14

-14*-14=196

Dog

5

5-22=-17

-17*-17=289

Total:

1426

Total/4:

356.5

> sd(c(43,42,12,8,5))

[1] 18.88121

This is a pretty boring example, though, and not very useful for the

rest of the chapter, so here’s the next step up in looking at data. We

will use the Windows or Mac clipboard to cut and paste a larger

data set into R. Go to the U.S. Census website where they have

stored population data:

http://www.census.gov/popest/data/national/totals/2011/inde

x.html

Assuming you have a spreadsheet program available, click on the

XLS link for "Annual Estimates of the Resident Population for the

United States." When the spreadsheet is open, select the population

estimates for the ﬁfty states. The ﬁrst few looked like this in the

2011 data:

.Alabama

4,779,736

.Alaska

710,231

.Arizona

6,392,017

.Arkansas

2,915,918

To make use of the next R command, make sure to choose just the

numbers and not the text. Before you copy the numbers, take out

the commas by switching the cell type to "General." This can usu-

ally be accomplished under the Format menu, but you might also

have a toolbar button to do the job. Copy the numbers to the clip-

board with ctrl+C (Windows) or command+C (Mac). On a Win-

dows machine use the following command:

read.DIF("clipboard",transpose=TRUE)

On a Mac, this command does the same thing:

read.table(pipe("pbpaste"))

It is very annoying that there are two different commands for the

two types of computers, but this is an inevitable side effect of the

different ways that the designers at Microsoft and Apple set up the

clipboard, plus the fact that R was designed to work across many

platforms. Anyway, you should have found that the long string of

population numbers appeared on the R output. The numbers are

not much use to us just streamed to the output, so let’s assign the

numbers to a new vector.

Windows, using read.DIF:

> USstatePops <- +

read.DIF("clipboard",transpose=TRUE)

> USstatePops

V1

1 4779736

2 710231

3 6392017

...

Or Mac, using read.table:

> USstatePops <- read.table(pipe("pbpaste"))

> USstatePops

41

V1

1 4779736

2 710231

3 6392017

...

Only the ﬁrst three observations are shown in order to save space

on this page. Your output R should show the whole list. Note that

the only thing new here over and above what we have done with R

in previous chapters is the use of the read.DIF() or read.table() func-

tions to get a bigger set of data that we don’t have to type our-

selves. Functions like read.table() are quite important as we move

forward with using R because they provide the usual way of get-

ting data stored in external ﬁles into R’s storage areas for use in

data analysis. If you had trouble getting this to work, you can cut

and paste the commands at the end of the chapter under "If All

Else Fails" to get the same data going in your copy of R.

Note that we have used the left pointing assignment arrow ("<-") to

take the results of the read.DIF() or read.table() function and place

it in a data object. This would be a great moment to practice your

skills from the previous chapter by using the str() and summary()

functions on our new data object called USstatePops. Did you no-

tice anything interesting from the results of these functions? One

thing you might have noticed is that there are 51 observations in-

stead of 50. Can you guess why? If not, go back and look at your

original data from the spreadsheet or the U.S. Census site. The

other thing you may have noticed is that USstatePops is a data-

frame, and not a plain vector of numbers. You can actually see this

in the output above: In the second command line where we request

that R reveal what is stored in USstatePops, it responds with a col-

umn topped by the designation "V1". Because we did not give R

any information about the numbers it read in from the clipboard, it

called them "V1", short for Variable One, by default. So anytime we

want to refer to our list of population numbers we actually have to

use the name USstatePops$V1. If this sounds unfamiliar, take an-

other look at the previous "Rows and Columns" chapter for more

information on addressing the columns in a dataframe.

Now we’re ready to have some fun with a good sized list of num-

bers. Here are the basic descriptive statistics on the population of

the states:

> mean(USstatePops$V1)

[1] 6053834

> median(USstatePops$V1)

[1] 4339367

> mode(USstatePops$V1)

[1] "numeric"

> var(USstatePops$V1)

[1] 4.656676e+13

> sd(USstatePops$V1)

[1] 6823984

Some great summary information there, but wait, a couple things

have gone awry:

42

•

The mode() function has returned the data type of our vector of

numbers instead of the statistical mode. This is weird but true:

the basic R package does not have a statistical mode function!

This is partly due to the fact that the mode is only useful in a

very limited set of situations, but we will ﬁnd out in later chap-

ters how add-on packages can be used to get new functions in R

including one that calculates the statistical mode.

•

The variance is reported as 4.656676e+13. This is the ﬁrst time

that we have seen the use of scientiﬁc notation in R. If you

haven’t seen this notation before, the way you interpret it is to

imagine 4.656676 multiplied by 10,000,000,000,000 (also known

as 10 raised to the 13th power). You can see that this is ten tril-

lion, a huge and unwieldy number, and that is why scientiﬁc no-

tation is used. If you would prefer not to type all of that into a

calculator, another trick to see what number you are dealing

with is just to move the decimal point 13 digits to the right.

Other than these two issues, we now know that the average popula-

tion of a U.S. state is 6,053,834 with a standard deviation of

6,823,984. You may be wondering, though, what does it mean to

have a standard deviation of almost seven million? The mean and

standard deviation are OK, and they certainly are mighty precise,

but for most of us, it would make much more sense to have a pic-

ture that shows the central tendency and the dispersion of a large

set of numbers. So here we go. Run this command:

hist(USstatePops$V1)

Here’s the output you should get:

Histogram of USstatePops$V1

USstatePops$V1

Frequency

0e+00 1e+07 2e+07 3e+07 4e+07

0 5 10 15 20 25 30

A histogram is a specialized type of bar graph designed to show

"frequencies." Frequencies means how often a particular value or

range of values occurs in a dataset. This histogram shows a very

interesting picture. There are nearly 30 states with populations un-

der ﬁve million, another 10 states with populations under 10 mil-

lion, and then a very small number of states with populations

greater than 10 million. Having said all that, how do we glean this

kind of information from the graph? First, look along the Y-axis

(the vertical axis on the left) for an indication of how often the data

43

occur. The tallest bar is just to the right of this and it is nearly up to

the 30 mark. To know what this tall bar represents, look along the

X-axis (the horizontal axis at the bottom) and see that there is a tick

mark for every two bars. We see scientiﬁc notation under each tick

mark. The ﬁrst tick mark is 1e+07, which translates to 10,000,000.

So each new bar (or an empty space where a bar would go) goes

up by ﬁve million in population. With these points in mind it

should now be easy to see that there are nearly 30 states with popu-

lations under ﬁve million.

If you think about presidential elections, or the locations of schools

and businesses, or how a single U.S. state might compare with

other countries in the world, it is interesting to know that there are

two really giant states and then lots of much smaller states. Once

you have some practice reading histograms, all of the knowledge is

available at a glance.

On the other hand there is something unsatisfying about this dia-

gram. With over forty of the states clustered into the ﬁrst couple of

bars, there might be some more details hiding in there that we

would like to know about. This concern translates into the number

of bars shown in the histogram. There are eight shown here, so

why did R pick eight?

The answer is that the hist() function has an algorithm or recipe for

deciding on the number of categories/bars to use by default. The

number of observations and the spread of the data and the amount

of empty space there would be are all taken into account. Fortu-

nately it is possible and easy to ask R to use more or fewer

categories/bars with the "breaks" parameter, like this:

hist(USstatePops$V1, breaks=20)

Histogram of USstatePops$V1

USstatePops$V1

Frequency

0e+00 1e+07 2e+07 3e+07

0 5 10 15

This gives us ﬁve bars per tick mark or about two million for each

bar. So the new histogram above shows very much the same pat-

tern as before: 15 states with populations under two million. The

pattern that you see here is referred to as a distribution. This is a

distribution that starts off tall on the left and swoops downward

quickly as it moves to the right. You might call this a "reverse-J" dis-

tribution because it looks a little like the shape a J makes, although

ﬂipped around vertically. More technically this could be referred to

as a Pareto distribution (named after the economist Vilfredo Pa-

44

reto). We don’t have to worry about why it may be a Pareto distri-

bution at this stage, but we can speculate on why the distribution

looks the way it does. First, you can’t have a state with no people

in it, or worse yet negative population. It just doesn’t make any

sense. So a state has to have at least a few people in it, and if you

look through U.S. history every state began as a colony or a terri-

tory that had at least a few people in it. On the other hand, what

does it take to grow really large in population? You need a lot of

land, ﬁrst of all, and then a good reason for lots of people to move

there or lots of people to be born there. So there are lots of limits to

growth: Rhode Island is too small too have a bazillion people in it

and Alaska, although it has tons of land, is too cold for lots of peo-

ple to want to move there. So all states probably started small and

grew, but it is really difﬁcult to grow really huge. As a result we

have a distribution where most of the cases are clustered near the

bottom of the scale and just a few push up higher and higher. But

as you go higher, there are fewer and fewer states that can get that

big, and by the time you are out at the end, just shy of 40 million

people, there’s only one state that has managed to get that big. By

the way, do you know or can you guess what that humongous

state is?

There are lots of other distribution shapes. The most common one

that almost everyone has heard of is sometimes called the "bell"

curve because it is shaped like a bell. The technical name for this is

the normal distribution. The term "normal" was ﬁrst introduced by

Carl Friedrich Gauss (1777-1855), who supposedly called it that in

a belief that it was the most typical distribution of data that one

might ﬁnd in natural phenomena. The following histogram depicts

the typical bell shape of the normal distribution.

Histogram of rnorm(51, 6053834, 6823984)

rnorm(51, 6053834, 6823984)

Frequency

-1e+07 0e+00 1e+07 2e+07 3e+07

0 5 10 15

If you are curious, you might be wondering how R generated the

histogram above, and, if you are alert, you might notice that the his-

togram that appears above has the word "rnorm" in a couple of

places. Here’s another of the cool features in R: it is incredibly easy

to generate "fake" data to work with when solving problems or giv-

45

ing demonstrations. The data in this histogram were generated by

R’s rnorm() function, which generates a random data set that ﬁts

the normal distribution (more closely if you generate a lot of data,

less closely if you only have a little. Some further explanation of

the rnorm() command will make sense if you remember that the

state population data we were using had a mean of 6,053,834 and a

standard deviation of 6,823,984. The command used to generate

this histogram was:

hist(rnorm(51, 6043834, 6823984))

There are two very important new concepts introduced here. The

ﬁrst is a nested function call: The hist() function that generates the

graph "surrounds" the rnorm() function that generates the new

fake data. (Pay close attention to the parentheses!) The inside func-

tion, rnorm(), is run by R ﬁrst, with the results of that sent directly

and immediately into the hist() function.

The other important thing is the "arguments that" were "passed" to

the rnorm() function. We actually already ran into arguments a lit-

tle while ago with the read.DIF() and read.table() functions but we

did not talk about them then. "Argument" is a term used by com-

puter scientists to refer to some extra information that is sent to a

function to help it know how to do its job. In this case we passed

three arguments to rnorm() that it was expecting in this order: the

number of observations to generate in the fake dataset, the mean of

the distribution, and the standard deviation of the distribution.

The rnorm() function used these three numbers to generate 51 ran-

dom data points that, roughly speaking, ﬁt the normal distribu-

tion. So the data shown in the histogram above are an approxima-

tion of what the distribution of state populations might look like if,

instead of being reverse-J-shaped (Pareto distribution), they were

normally distributed.

The normal distribution is used extensively through applied statis-

tics as a tool for making comparisons. For example, look at the

rightmost bar in the previous histogram. The label just to the right

of that bar is 3e+07, or 30,000,000. We already know from our real

state population data that there is only one actual state with a

population in excess of 30 million (if you didn’t look it up, it is Cali-

fornia). So if all of a sudden, someone mentioned to you that he or

she lived in a state, other than California, that had 30 million peo-

ple, you would automatically think to yourself, "Wow, that’s un-

usual and I’m not sure I believe it." And the reason that you found

it hard to believe was that you had a distribution to compare it to.

Not only did that distribution have a characteristic shape (for exam-

ple, J-shaped, or bell shaped, or some other shape), it also had a

center point, which was the mean, and a "spread," which in this

case was the standard deviation. Armed with those three pieces of

information - the type/shape of distribution, an anchoring point,

and a spread (also known as the amount of variability), you have a

powerful tool for making comparisons.

In the next chapter we will conduct some of these comparisons to

see what we can infer about the ways things are in general, based

on just a subset of available data, or what statisticians call a sam-

ple.

Chapter Challenge

In this chapter, we used rnorm() to generate random numbers that

closely ﬁt a normal distribution. We also learned that the state

population data was a "Pareto" distribution. Do some research to

46

ﬁnd out what R function generates random numbers using the Pa-

reto distribution. Then run that function with the correct parame-

ters to generate 51 random numbers (hint: experiment with differ-

ent probability values). Create a histogram of these random num-

bers and describe the shape of the distribution.

Sources

http://en.wikipedia.org/wiki/Carl_Friedrich_Gauss

http://en.wikipedia.org/wiki/Francis_Galton

http://en.wikipedia.org/wiki/Pareto_distribution

http://en.wikipedia.org/wiki/Karl_Pearson

http://en.wikipedia.org/wiki/Ronald_Fisher

http://en.wikipedia.org/wiki/William_Sealy_Gosset

http://en.wikipedia.org/wiki/Normal_distribution

http://stat.ethz.ch/R-manual/R-devel/library/utils/html/read.t

able.html

http://www.census.gov/popest/data/national/totals/2011/inde

x.html

http://www.r-tutor.com/elementary-statistics/numerical-measur

es/standard-deviation

47

Review 6.1 Beer, Farms, and Peas

Check Answer

A bar graph that displays the frequencies of occurrence for a

numeric variable is called a

A.

Histogram

B.

Pictogram

C. Bar Graph

D. Bar Chart

R Functions Used in This Chapter

read.DIF()!! Reads data in interchange format

read.table()! Reads data table from external source

mean()! ! Calculate arithmetic mean

median()! ! Locate the median

mode()! ! Tells the data type/mode of a data object!

! ! ! Note: This is NOT the statistical mode

var()!! ! Calculate the sample variance

sd()! ! ! Calculate the sample standard deviation

hist()!! ! Produces a histogram graphic

Test Yourself

If All Else Fails

In case you have difﬁculty with the read.DIF() or read.table() func-

tions, the code shown below can be copied and pasted (or, in the

worst case scenario, typed) into the R console to create the data set

used in this chapter.

V1 <- c(4779736,710231,6392017,2915918,37253956, "

5029196,3574097,897934,601723,18801310,9687653, "

1360301,1567582,12830632,6483802,3046355,2853118,

4339367,4533372,1328361,5773552,6547629,9883640, "

5303925,2967297,5988927,989415,1826341,2700551, "

1316470,8791894,2059179,19378102,9535483,672591, "

11536504,3751351,3831074,12702379,1052567, "

4625364,814180,6346105,25145561,2763885,625741, "

8001024,6724540,1852994,5686986,563626)

USstatePops <- data.frame(V1)

48

Sampling distributions are the conceptual key to statistical inference. Many approaches to

understanding sampling distributions use examples of drawing marbles or gumballs from a large jar

to illustrate the inﬂuences of randomness on sampling. Using the list of U.S. states illustrates how a

non-normal distribution nonetheless has a normal sampling distribution of means.

CHAPTER 7

49

Sample in a Jar

Imagine a gum ball jar full of gumballs of two different colors, red

and blue. The jar was ﬁlled from a source that provided 100 red

gum balls and 100 blue gum balls, but when these were poured

into the jar they got all mixed up. If you drew eight gumballs from

the jar at random, what colors would you get? If things worked out

perfectly, which they never do, you would get four red and four

blue. This is half and half, the same ratio of red and blue that is in

the jar as a whole. Of course, it rarely works out this way, does it?

Instead of getting four red and four blue you might get three red

and ﬁve blue or any other mix you can think of. In fact, it would be

possible, though perhaps not likely, to get eight red gumballs. The

basic situation, though, is that we really don’t know what mix of

red and blue we will get with one draw of eight gumballs. That’s

uncertainty for you, the forces of randomness affecting our sample

of eight gumballs in unpredictable ways.

Here’s an interesting idea, though, that is no help at all in predict-

ing what will happen in any one sample, but is great at showing

what will occur in the long run. Pull eight gumballs from the jar,

count the number of red ones and then throw them back. We do

not have to count the number of blue because 8 - #red = #blue. Mix

up the jar again and then draw eight more gumballs and count the

number of red. Keeping doing this many times. Here’s an example

of what you might get:

Notice that the left column is just counting up the number of sam-

ple draws we have done. The right column is the interesting one

because it is the count of the number of red gumballs in each par-

ticular sample draw. In this example, things are all over the place.

In sample draw 4 we only have two red gumballs, but in sample

draw 3 we have 6 red gumballs. But the most interesting part of

this example is that if you average the number of red gumballs over

all of the draws, the average comes out to exactly four red gumballs

per draw, which is what we would expect in a jar that is half and

half. Now this is a contrived example and we won’t always get

such a perfect result so quickly, but if you did four thousand draws

instead of four, you would get pretty close to the perfect result.

This process of repeatedly drawing a subset from a "population" is

called "sampling," and the end result of doing lots of sampling is a

sampling distribution. Note that we are using the word population

in the previous sentence in its statistical sense to refer to the total-

ity of units from which a sample can be drawn. It is just a coinci-

dence that our dataset contains the number of people in each state

and that this value is also referred to as "population." Next we will

get R to help us draw lots of samples from our U.S. state dataset.

Conveniently, R has a function called sample(), that will draw a ran-

dom sample from a data set with just a single call. We can try it

now with our state data:

> sample(USstatePops$V1,size=16,replace=TRUE)

[1] 4533372 19378102 897934 1052567 672591

18801310 2967297

[8] 5029196

50

DRAW

# RED

1

5

2

3

3

6

4

2

As a matter of practice, note that we called the sample() function

with three arguments. The ﬁrst argument was the data source. For

the second and third arguments, rather than rely on the order in

which we specify the arguments, we have used "named argu-

ments" to make sure that R does what we wanted. The size=16 ar-

gument asks R to draw a sample of 16 state data values. The repla-

ce=TRUE argument speciﬁes a style of sampling which statisticians

use very often to simplify the mathematics of their proofs. For us,

sampling with or without replacement does not usually have any

practical effects, so we will just go with what the statisticians typi-

cally do.

When we’re working with numbers such as these state values, in-

stead of counting gumball colors, we’re more interested in ﬁnding

out the average, or what you now know as the mean. So we could

also ask R to calculate a mean() of the sample for us:

> mean(sample(USstatePops$V1,size=16, +"

replace=TRUE))

[1] 8198359

There’s the nested function call again. The output no longer shows

the 16 values that R sampled from the list of 51. Instead it used

those 16 values to calculate the mean and display that for us. If you

have a good memory, or merely took the time to look in the last

chapter, you will remember that the actual mean of our 51 observa-

tions is 6,053,834. So the mean that we got from this one sample of

16 states is really not even close to the true mean value of our 51

observations. Are we worried? Deﬁnitely not! We know that when

we draw a sample, whether it is gumballs or states, we will never

hit the true population mean right on the head. We’re interested

not in any one sample, but in what happens over the long haul. So

now we’ve got to get R to repeat this process for us, not once, not

four times, but four hundred times or four thousand times. Like

most programming languages, R has a variety of ways of repeating

an activity. One of the easiest ones to use is the replicate() function.

To start, let’s just try four replications:

> replicate(4, mean(sample(USstatePops$V1,+

size=16,replace=TRUE)),simplify=TRUE)

[1] 10300486 11909337 8536523 5798488

Couldn’t be any easier. We took the exact same command as be-

fore, which was a nested function to calculate the mean() of a ran-

dom sample of 16 states (shown above in bold). This time, we put

that command inside the replicate() function so we could run it

over and over again. The simplify=TRUE argument asks R to re-

turn the results as a simple vector of means, perfect for what we

are trying to do. We only ran it four times, so that we would not

have a big screen full of numbers. From here, though, it is easy to

ramp up to repeating the process four hundred times. You can try

that and see the output, but for here in the book we will encapsu-

late the whole replicate function inside another mean(), so that we

can get the average of all 400 of the sample means. Here we go:

> mean(replicate(400, mean( + "

sample(USstatePops$V1,size=16,replace=TRUE)),+"

simplify=TRUE))

[1] 5958336

In the command above, the outermost mean() command is bolded

to show what is different from the previous command. So, put into

51

words, this deeply nested command accomplishes the following: a)

Draw 400 samples of size n=8 from our full data set of 51 states; b)

Calculate the mean from each sample and keep it in a list; c) When

ﬁnished with the list of 400 of these means, calculate the mean of

that list of means. You can see that the mean of four hundred sam-

ple means is 5,958,336. Now that is still not the exact value of the

whole data set, but it is getting close. We’re off by about 95,000,

which is roughly an error of about 1.6% (more precisely, 95,498/

6,053,834 = 1.58%. You may have also noticed that it took a little

while to run that command, even if you have a fast computer.

There’s a lot of work going on there! Let’s push it a bit further and

see if we can get closer to the true mean for all of our data:

> mean(replicate(4000, mean( +"

sample(USstatePops$V1,size=16,replace=TRUE)),+"

simplify=TRUE))

[1] 6000972

Now we are even closer! We are now less than 1% away from the

true population mean value. Note that the results you get may be a

bit different, because when you run the commands, each of the 400

or 4000 samples that is drawn will be slightly different than the

ones that were drawn for the commands above. What will not be

much different is the overall level of accuracy.

We’re ready to take the next step. Instead of summarizing our

whole sampling distribution in a single average, let’s look at the

distribution of means using a histogram.

The histogram displays the complete list of 4000 means as frequen-

cies. Take a close look so that you can get more practice reading fre-

quency histograms. This one shows a very typical conﬁguration

that is almost bell-shaped, but still has a bit of "skewness" off to the

right. The tallest, and therefore most frequent range of values is

right near the true mean of 6,053,834.

Histogram of replicate(4000, mean(sample(USstatePops$V1, size = 16, replace = TRUE)), simplify = TRUE)

replicate(4000, mean(sample(USstatePops$V1, size = 16, replace = TRUE)), simplify = TRUE)

Frequency

2.0e+06 4.0e+06 6.0e+06 8.0e+06 1.0e+07 1.2e+07 1.4e+07

0 200 400 600 800 1000

By the way, were you able to ﬁgure out the command to generate

this histogram on your own? All you had to do was substitute

hist() for the outermost mean() in the previous command. In case

you struggled, here it is:

52

hist(replicate(4000, mean( +"

sample(USstatePops$V1,size=16,replace=TRUE)), +"

simplify=TRUE))

This is a great moment to take a deep breath. We’ve just covered a

couple hundred years of statistical thinking in just a few pages. In

fact, there are two big ideas, "the law of large numbers" and !

the central limit theorem" that we have just partially demonstrated.

These two ideas literally took mathematicians like Gerolamo Car-

dano (1501-1576) and Jacob Bernoulli (1654-1705) several centuries

to ﬁgure out. If you look these ideas up, you may ﬁnd a lot of be-

wildering mathematical details, but for our purposes, there are two

really important take-away messages. First, if you run a statistical

process a large number of times, it will converge on a stable result.

For us, we knew what the average population was of the 50 states

plus the District of Columbia. These 51 observations were our

population, and we wanted to know how many smaller subsets, or

samples, of size n=16 we would have to draw before we could get

a good approximation of that true value. We learned that drawing

one sample provided a poor result. Drawing 400 samples gave us a

mean that was off by 1.5%. Drawing 4000 samples gave us a mean

that was off by less than 1%. If we had kept going to 40,000 or

400,000 repetitions of our sampling process, we would have come

extremely close to the actual average of 6,053,384.

Second, when we are looking at sample means, and we take the

law of large numbers into account, we ﬁnd that the distribution of

sampling means starts to create a bell-shaped or normal distribu-

tion, and the center of that distribution, the mean of all of those

sample means gets really close to the actual population mean. It

gets closer faster for larger samples, and in contrast, for smaller

samples you have to draw lots and lots of them to get really close.

Just for fun, lets illustrate this with a sample size that is larger than

16. Here’s a run that only repeats 100 times, but each time draws a

sample of n=51 (equal in size to the population):

> mean(replicate(100, mean( + "

sample(USstatePops$V1,size=51,replace=TRUE)),+"

simplify=TRUE))

[1] 6114231

Now, we’re only off from the true value of the population mean by

about one tenth of one percent. You might be scratching your head

now, saying, "Wait a minute, isn’t a sample of 51 the same thing as

the whole list of 51 observations?" This is confusing, but it goes

back to the question of sampling with replacement that we exam-

ined a couple of pages ago (and that appears in the command

above as replace=TRUE). Sampling with replacement means that

as you draw out one value to include in your random sample, you

immediately chuck it back into the list so that, potentially, it could

get drawn again either immediately or later. As mentioned before,

this practice simpliﬁes the underlying proofs, and it does not cause

any practical problems, other than head scratching. In fact, we

could go even higher in our sample size with no trouble:

> mean(replicate(100, mean( +"

sample(USstatePops$V1,size=120,replace=TRUE)), +"

simplify=TRUE))

[1] 6054718

That command runs 100 replications using samples of size n=120.

Look how close the mean of the sampling distribution is to the

53

population mean now! Remember that this result will change a lit-

tle bit every time you run the procedure, because different random

samples are being drawn for each run. But the rule of thumb is that

the bigger your sample size, what statisticians call n, the closer

your estimate will be to the true value. Likewise, the more trials

you run, the closer your population estimate will be.

So, if you’ve had a chance to catch your breath, let’s move on to

making use of the sampling distribution. First, let’s save one distri-

bution of sample means so that we have a ﬁxed set of numbers to

work with:

SampleMeans <- replicate(10000, mean(sample(US-

statePops$V1,size=5,+"

replace=TRUE)),simplify=TRUE)

The bolded part is new. We’re saving a distribution of sample

means to a new vector called "SampleMeans". We should have

10,000 of them:

> length(SampleMeans)

[1] 10000

And the mean of all of these means should be pretty close to our

population mean of 6,053,384:

> mean(SampleMeans)

[1] 6065380

You might also want to run a histogram on SampleMeans and see

what the frequency distribution looks like. Right now, all we need

to look at is a summary of the list of sample means:

> summary(SampleMeans)

Min. 1st Qu. Median Mean 3rd Qu. Max.

799100 3853000 5370000 6065000 7622000 25030000

If you need a refresher on the median and quartiles, take a look

back at Chapter 3 - Rows and Columns.

This summary is full of useful information. First, take a look at the

max and the min. The minimum sample mean in the list was

799,100. Think about that for a moment. How could a sample have

a mean that small when we know that the true mean is much

higher? Rhode Island must have been drawn several times in that

sample! The answer comes from the randomness involved in sam-

pling. If you run a process 10,000 times you are deﬁnitely going to

end up with a few weird examples. Its almost like buying a lottery

ticket. The vast majority of tickets are the usual - not a winner.

Once in a great while, though, there is a very unusual ticket - a win-

ner. Sampling is the same: The extreme events are unusual, but

they do happen if you run the process enough times. The same

goes for the maximum: at 25,030,000 the maximum sample mean is

much higher than the true mean.

At 5,370,000 the median is quite close to the mean, but not exactly

the same because we still have a little bit of rightward skew (the

"tail" on the high side is slightly longer than it should be because of

the reverse J-shape of the original distribution). The median is very

useful because it divides the sample exactly in half: 50%, or exactly

5000 of the sample means are larger than 5,370,000 and the other

50% are lower. So, if we were to draw one more sample from the

population it would have a ﬁfty-ﬁfty chance of being above the me-

dian. The quartiles help us to cut things up even more ﬁnely. The

54

third quartile divides up the bottom 75% from the top 25%. So only

25% of the sample means are higher than 7,622,000. That means if

we drew a new sample from the population that there is only a

25% chance that it will be larger than that. Likewise, in the other

direction, the ﬁrst quartile tells us that there is only a 25% chance

that a new sample would be less than 3,853,000.

There is a slightly different way of getting the same information

from R that will prove more ﬂexible for us in the long run. The

quantile() function can show us the same information as the me-

dian and the quartiles, like this:

> quantile(SampleMeans, probs=c(0.25,0.50,0.75))

25% 50% 75%

3853167 5370314 7621871

You will notice that the values are just slightly different, by less

than one tenth of one percent, than those produced by the sum-

mary() function. These are actually more precise, although the less

precise ones from summary() are ﬁne for most purposes. One rea-

son to use quantile() is that it lets us control exactly where we

make the cuts. To get quartiles, we cut at 25% (0.25 in the com-

mand just above), at 50%, and at 75%. But what if we wanted in-

stead to cut at 2.5% and 97.5%? Easy to do with quantile():

> quantile(SampleMeans, probs=c(0.025,0.975))

2.5% 97.5%

2014580 13537085

So this result shows that, if we drew a new sample, there is only a

2.5% chance that the mean would be lower than 2,014,580. Like-

wise, there is only a 2.5% chance that the new sample mean would

be higher than 13,537,085 (because 97.5% of the means in the sam-

pling distribution are lower than that value).

Now let’s put this knowledge to work. Here is a sample of the num-

ber of people in a certain area, where each of these areas is some

kind of a unit associated with the U.S.:

3,706,690!

159,358!

106,405!

55,519!

53,883

We can easily get these into R and calculate the sample mean:

> MysterySample <- c(3706690, 159358, 106405, +"

55519, 53883)

> mean(MysterySample)

[1] 816371

The mean of our mystery sample is 816,371. The question is, is this

a sample of U.S. states or is it something else? Just on its own it

would be hard to tell. The ﬁrst observation in our sample has more

people in it than Kansas, Utah, Nebraska, and several other states.

We also know from looking at the distribution of raw population

data from our previous example that there are many, many states

that are quite small in the number of people. Thanks to the work

we’ve done earlier in this chapter, however, we have an excellent

basis for comparison. We have the sampling distribution of means,

and it is fair to say that if we get a new mean to look at, and the

new mean is way out in the extreme areas of the sample distribu-

55

tion, say, below the 2.5% mark or above the 97.5% mark, then it

seems much less likely that our MysterySample is a sample of

states.

In this case, we can see quite clearly that 816,371 is on the extreme

low end of the sampling distribution. Recall that when we ran the

quantile() command we found that only 2.5% of the sample means

in the distribution were smaller than 2,014,580.

In fact, we could even play around with a more stringent criterion:

> quantile(SampleMeans, probs=c(0.005,0.995))

0.5% 99.5%

1410883 16792211

This quantile() command shows that only 0.5% of all the sample

means are lower than 1,410,883. So our MysterySample mean of

816,371 would deﬁnitely be a very rare event, if it were truly a sam-

ple of states. From this we can infer, tentatively but based on good

statistical evidence, that our MysterySample is not a sample of

states. The mean of MysterySample is just too small to be very

likely to be a sample of states.

And this is in fact correct: MysterySample contains the number of

people in ﬁve different U.S. territories, including Puerto Rico in the

Caribbean and Guam in the Paciﬁc. These territories are land

masses and groups of people associated with the U.S., but they are

not states and they are different in many ways than states. For one

thing they are all islands, so they are limited in land mass. Among

the U.S. states, only Hawaii is an island, and it is actually bigger

than 10 of the states in the continental U.S. The important thing to

take away is that the characteristics of this group of data points, no-

tably the mean of this sample, was sufﬁciently different from a

known distribution of means that we could make an inference that

the sample was not drawn from the original population of data.

This reasoning is the basis for virtually all statistical inference. You

construct a comparison distribution, you mark off a zone of ex-

treme values, and you compare any new sample of data you get to

the distribution to see if it falls in the extreme zone. If it does, you

tentatively conclude that the new sample was obtained from some

other source than what you used to create the comparison distribu-

tion.

If you feel a bit confused, take heart. There’s 400-500 years of

mathematical developments represented in that one preceding

paragraph. Also, before we had cool programs like R that could be

used to create and analyze actual sample distributions, most of the

material above was taught as a set of formulas and proofs. Yuck!

Later in the book we will come back to speciﬁc statistical proce-

dures that use the reasoning described above. For now, we just

need to take note of three additional pieces of information.

First, we looked at the mean of the sampling distribution with

mean() and we looked at its shaped with hist(), but we never quan-

tiﬁed the spread of the distribution:

> sd(SampleMeans)

[1] 3037318

This shows us the standard deviation of the distribution of sam-

pling means. Statisticians call this the "standard error of the mean."

This chewy phrase would have been clearer, although longer, if it

had been something like this: "the standard deviation of the distri-

56

bution of sample means for samples drawn from a population." Un-

fortunately, statisticians are not known for giving things clear la-

bels. Sufﬁce to say that when we are looking at a distribution and

each data point in that distribution is itself a representation of a

sample (for example, a mean), then the standard deviation is re-

ferred to as the standard error.

Second, there is a shortcut to ﬁnding out the standard error that

does not require actually constructing an empirical distribution of

10,000 (or any other number) of sampling means. It turns out that

the standard deviation of the original raw data and the standard

error are closely related by a simple bit of algebra:

> sd(USstatePops$V1)/sqrt(5)

[1] 3051779

The formula in this command takes the standard deviation of the

original state data and divides it by the square root of the sample

size. Remember three of four pages ago when we created the Sam-

pleMeans vector by using the replicate() and sample() commands,

that we used a sample size of n=5. That’s what you see in the for-

mula above, inside of the sqrt() function. In R, and other software

sqrt() is the abbreviation for "square root" and not for "squirt" as

you might expect. So if you have a set of observations and you cal-

culate their standard deviation, you can also calculate the standard

error for a distribution of means (each of which has the same sam-

ple size), just by dividing by the square root of the sample size. You

may notice that the number we got with the shortcut was slightly

larger than the number that came from the distribution itself, but

the difference is not meaningful (and only arrises because of ran-

domness in the distribution). Another thing you may have noticed

is that the larger the sample size, the smaller the standard error.

This leads to an important rule for working with samples: the big-

ger the better.

The last thing is another shortcut. We found out the 97.5% cut

point by constructing the sampling distribution and then using

quantile to tell us the actual cuts. You can also cut points just using

the mean and the standard error. Two standard errors down from

the mean is the 2.5% cut point and two standard errors up from the

mean is the 97.5% cut point.

> StdError<-sd(USstatePops$V1)/sqrt(5)

> CutPoint975<-mean(USstatePops$V1)+(2 * StdEr-

ror)

> CutPoint975

[1] 12157391

You will notice again that this value is different from what we cal-

culated with the quantile() function using the empirical distribu-

tion. The differences arise because of the randomness in the distri-

bution that we constructed. The value above is an estimate that is

based on statistical proofs, whereas the empirical SampleMeans list

that we constructed is just one of a nearly inﬁnite range of such

lists that we could create. We could easily reduce the discrepancy

between the two methods by using a larger sample size and by hav-

ing more replications included in the sampling distribution.

To summarize, with a data set that includes 51 data points with the

numbers of people in states, and a bit of work using R to construct

a distribution of sampling means, we have learned the following:

57

•

Run a statistical process a large number of times and you get a

consistent pattern of results.

•

Taking the means of a large number of samples and plotting

them on a histogram shows that the sample means are fairly

well normally distributed and that the center of the distribution

is very, very close to the mean of the original raw data.

•

This resulting distribution of sample means can be used as a ba-

sis for comparisons. By making cut points at the extreme low

and high ends of the distribution, for example 2.5% and 97.5%,

we have a way of comparing any new information we get.

•

If we get a new sample mean, and we ﬁnd that it is in the ex-

treme zone deﬁned by our cut points, we can tentatively con-

clude that the sample that made that mean is a different kind of

thing than the samples that made the sampling distribution.

•

A shortcut and more accurate way of ﬁguring the cut points in-

volves calculating the "standard error" based on the standard de-

viation of the original raw data.

We’re not statisticians at this point, but the process of reasoning

based on sampling distributions is at the heart of inferential statis-

tics, so if you have followed the logic presented in this chapter, you

have made excellent progress towards being a competent user of

applied statistics.

Chapter Challenge

Collect a sample consisting of at least 20 data points and construct

a sampling distribution. Calculate the standard error and use this

to calculate the 2.5% and 97.5% distribution cut points. The data

points you collect should represent instances of the same phenome-

non. For instance, you could collect the prices of 20 textbooks, or

count the number of words in each of 20 paragraphs.

Sources

http://en.wikipedia.org/wiki/Central_limit_theorem

http://en.wikipedia.org/wiki/Gerolamo_Cardano

http://en.wikipedia.org/wiki/Jacob_Bernoulli

http://en.wikipedia.org/wiki/Law_of_large_numbers

http://en.wikipedia.org/wiki/List_of_U.S._states_and_territories

_by_population

http://www.khanacademy.org/math/statistics/v/central-limit-th

eorem

R Commands Used in This Chapter

length() - The number of elements in a vector

mean() - The arithmetic mean or average of a set of values

quantile() - Calculates cut points based on percents/proportions

replicate() - Runs an expression/calculation many times

sample() - Chooses elements at random from a vector

sd() - Calculates standard deviation

58

sqrt() - Calculates square root

summary() - Summarizes contents of a vector

59

In 2012 the technology press contained many headlines about big data. What makes data big, and

why is this bigness important? In this chapter, we discuss some of the real issues behind these

questions. Armed with information from the previous chapter concerning sampling, we can give

more thought to how the size of a data set affects what we do with the data.

CHAPTER 8

60

Big Data? Big Deal!

MarketWatch (a Wall Street Journal Service) recently published an

article with the title, "Big Data Equals Big Business Opportunity

Say Global IT and Business Professionals," and the subtitle, "70 Per-

cent of Organizations Now Considering, Planning or Running Big

Data Projects According to New Global Survey." The technology

news has been full of similar articles for several years. Given the

number of such articles it is hard to resist the idea that "big data"

represents some kind of revolution that has turned the whole

world of information and technology topsy-turvy. But is this really

true? Does "big data" change everything?

Business analyst Doug Laney suggested that three characteristics

make "big data" different from what came before: volume, velocity,

and variety. Volume refers to the sheer amount of data. Velocity fo-

cuses on how quickly data arrives as well as how quickly those

data become "stale." Finally, Variety reﬂects the fact that there may

be many different kinds of data. Together, these three characteris-

tics are often referred to as the "three Vs" model of big data. Note,

however, that even before the dawn of the computer age we’ve had

a variety of data, some of which arrives quite quickly, and that can

add up to quite a lot of total storage over time (think, for example,

of the large variety and volume of data that has arrived annually at

Library of Congress since the 1800s!). So it is difﬁcult to tell, just

based on someone saying that they have a high volume, high veloc-

ity, and high variety data problem, that big data is fundamentally a

brand new thing.

With that said, there are certainly many changes afoot that make

data problems qualitatively different today as compared with a

few years ago. Let’s list a few things which are pretty accurate:

1. The decline in the price of sensors (like barcode readers) and

other technology over recent decades has made it cheaper and

easier to collect a lot more data.

2. Similarly, the declining cost of storage has made it practical to

keep lots of data hanging around, regardless of its quality or use-

fulness.

3. Many people’s attitudes about privacy seem to have accommo-

dated the use of Facebook and other platforms where we reveal

lots of information about ourselves.

4. Researchers have made signiﬁcant advances in the "machine

learning" algorithms that form the basis of many data mining

techniques.

5. When a data set gets to a certain size (into the range of thou-

sands of rows), conventional tests of statistical signiﬁcance are

meaningless, because even the most tiny and trivial results (or

effect sizes, as statisticians call them) are statistically signiﬁcant.

Keeping these points in mind, there are also a number of things

that have not changed throughout the years:

A. Garbage in, garbage out: The usefulness of data depends heav-

ily upon how carefully and well it was collected. After data were

collected, the quality depends upon how much attention was paid

to suitable pre-processing: data cleaning and data screening.

B. Bigger equals weirder: If you are looking for anomalies - rare

events that break the rules - then larger is better. Low frequency

events often do not appear until a data collection goes on for a long

61

time and/or encompasses a large enough group of instances to con-

tain one of the bizarre cases.

C. Linking adds potential: Standalone datasets are inherently lim-

ited by whatever variables are available. But if those data can be

linked to some other data, all of a sudden new vistas may open up.

No guarantees, but the more you can connect records here to other

records over there, the more potential ﬁndings you have.

Items on both of the lists above are considered pretty common-

place and uncontroversial. Taken together, however, they do shed

some light on the question of how important "big data" might be.

We have had lots of historical success using conventional statistics

to examine modestly sized (i.e., 1000 rows or less) datasets for sta-

tistical regularities. Everyone’s favorite basic statistic, the Student’s

t-test, is essential a test for differences in the central tendency of

two groups. If the data contain regularities such that one group is

notably different from another group, a t-test shows it to be so.

Big data does not help us with these kinds of tests. We don’t even

need a thousand records for many conventional statistical compari-

sons, and having a million or a hundred million records won’t

make our job any easier (it will just take more computer memory

and storage). Think about what you read in the previous chapter:

We were able to start using a basic form of statistical inference with

a data set that contained a population with only 51 elements. In

fact, many of the most commonly used statistical techniques, like

the Student’s t-test, were designed speciﬁcally to work with very

small samples.

On the other hand, if we are looking for needles in haystacks, it

makes sense to look (as efﬁciently as possible) through the biggest

possible haystack we can ﬁnd, because it is much more likely that a

big haystack will contain at least one needle and maybe more.

Keeping in mind the advances in machine learning that have oc-

curred over recent years, we begin to have an idea that good tools

together with big data and interesting questions about unusual pat-

terns could indeed provide some powerful new insights.

Let’s couple this optimism with three very important cautions. The

ﬁrst caution is that the more complex our data are, the more difﬁ-

cult it will be to ensure that the data are "clean" and suitable for the

purpose we plan for them. A dirty data set is worse in some ways

than no data at all because we may put a lot of time and effort into

ﬁnding an insight and ﬁnd nothing. Even more problematic, we

may put a lot of time and effort and ﬁnd a result that is simply

wrong! Many analysts believe that cleaning data - getting it ready

for analysis, weeding out the anomalies, organizing the data into a

suitable conﬁguration - actually takes up most of the time and ef-

fort of the analysis process.

The second caution is that rare and unusual events or patterns are

almost always by their nature highly unpredictable. Even with the

best data we can imagine and plenty of variables, we will almost

always have a lot of trouble accurately enumerating all of the

causes of an event. The data mining tools may show us a pattern,

and we may even be able to replicate the pattern in some new data,

but we may never be conﬁdent that we have understood the pat-

tern to the point where we believe we can isolate, control, or under-

stand the causes. Predicting the path of hurricanes provides a great

example here: despite decades of advances in weather instrumenta-

tion, forecasting, and number crunching, meteorologists still have

great difﬁculty predicting where a hurricane will make landfall or

62

how hard the winds will blow when it gets there. The complexity

and unpredictability of the forces at work make the task exceed-

ingly difﬁcult.

The third caution is about linking data sets. Item C above suggests

that linkages may provide additional value. With every linkage to

a new data set, however, we also increase the complexity of the

data and the likelihood of dirty data and resulting spurious pat-

terns. In addition, although many companies seem less and less

concerned about the idea, the more we link data about living peo-

ple (e.g., consumers, patients, voters, etc.) the more likely we are to

cause a catastrophic loss of privacy. Even if you are not a big fan of

the importance of privacy on principle, it is clear that security and

privacy failures have cost companies dearly both in money and

reputation. Today’s data innovations for valuable and acceptable

purposes maybe tomorrow’s crimes and scams. The greater the

amount of linkage between data sets, the easier it is for those peo-

ple with malevolent intentions to exploit it.

Putting this altogether, we can take a sensible position that high

quality data, in abundance, together with tools used by intelligent

analysts in a secure environment, may provide worthwhile bene-

ﬁts in the commercial sector, in education, in government, and in

other areas. The focus of our efforts as data scientists, however,

should not be on achieving the largest possible data sets, but rather

on getting the right data and the right amount of data for the pur-

pose we intend. There is no special virtue in having a lot of data if

those data are unsuitable to the conclusions that we want to draw.

Likewise, simply getting data more quickly does not guarantee

that what we get will be highly relevant to our problems. Finally,

although it is said that variety is the spice of life, complexity is of-

ten a danger to reliability and trustworthiness: the more complex

the linkages among our data the more likely it is that problems

may crop up in making use of those data or keeping them safe.

The Tools of Data Science

Over the past few chapters, we’ve gotten a pretty quick jump start

on an analytical tool used by thousands of data analysts world-

wide - the open source R system for data analysis and visualiza-

tion. Despite the many capabilities of R, however, there are hun-

dreds of other tools used by data scientists, depending on the par-

ticular aspects of the data problem they focus on.

The single most popular and powerful tool, outside of R, is a pro-

prietary statistical system called SAS (pronounced "sass"). SAS con-

tains a powerful programming language that provides access to

many data types, functions, and language features. Learning SAS

is arguably as difﬁcult (or as easy, depending upon your perspec-

tive) as learning R, but SAS is used by many large corporations be-

cause, unlike R, there is extensive technical and product support

on offer. Of course, this support does not come cheap, so most SAS

users work in large organizations that have sufﬁcient resources to

purchase the necessary licenses and support plans.

Next in line in the statistics realm is SPSS, a package used by many

scientists (the acronym used to stand for Statistical Package for the

Social Sciences). SPSS is much friendlier than SAS, in the opinion

of many analysts, but not quite as ﬂexible and powerful.

R, SPSS, and SAS grew up as statistics packages, but there are also

many general purpose programming languages that incorporate

features valuable to data scientists. One very exciting development

63

in programming languages has the odd name of "Processing." Proc-

essing is a programming language speciﬁcally geared toward creat-

ing data visualizations. Like R, Processing is an open source pro-

ject, so it is freely available at http://processing.org/. Also like R,

Processing is a cross-platform program, so it will run happily on

Mac, Windows, and Linux. There are lots of books available for

learning Processing (unfortunately, no open source books yet) and

the website contains lots of examples for getting started. Besides R,

Processing might be one of the most important tools in the data sci-

entist’s toolbox, at least for those who need to use data to draw con-

clusions and communicate with others.

Chapter Challenge

Look over the various websites connected with "Data.gov" to ﬁnd

the largest and/or most complex data set that you can. Think

about (and perhaps write about) one or more of the ways that

those data could potentially be misused by analysts. Download a

data set that you ﬁnd interesting and read it into R to see what you

can do with it.

For a super extra challenge, go to this website:

http://teamwpc.co.uk/products/wps

and download a trial version of the "World Programming System"

(WPS). WPS can read SAS code, so you could easily look up the

code that you would need in order to read in your Data.gov data-

set.

Sources

http://aqua.nasa.gov/doc/pubs/Wx_Forecasting.pdf

http://en.wikipedia.org/wiki/Big_data

http://en.wikipedia.org/wiki/Data.gov

http://www.marketwatch.com/story/big-data-equals-big-busines

s-opportunity-say-global-it-and-business-professionals-2012-05-14

64

As an open source program with an active user community, R enjoys constant innovation thanks to

the dedicated developers who work on it. One useful innovation was the development of R-Studio, a

beautiful frame to hold your copy of R. This chapter walks through the installation of R-Studio and

introduces "packages," the key to the extensibility of R.

CHAPTER 9

65

Onward with R-Studio

Joseph J. Allaire is a serial entrepreneur, software engineer, and the

originator of some remarkable software products including "Cold-

Fusion," which was later sold to the web media tools giant Mac-

romedia and Windows Live Writer, a Microsoft blogging tool. Start-

ing in 2009, Allaire began working with a small team to develop an

open source program that enhances the usability and power of R.

As mentioned in previous chapters, R is an open source program,

meaning that the source code that is used to create a copy of R to

run on a Mac, Windows, or Linux computer is available for all to

inspect and modify. As with many open source projects, there is an

active community of developers who work on R, both on the basic

program itself and the many pieces and parts that can be added

onto the basic program. One of these add-ons is R-Studio. R-Studio

is an Integrated Development Environment, abbreviated as IDE.

Every software engineer knows that if you want to get serious

about building something out of code, you must use an IDE. If you

think of R as a piece of canvas rolled up and laying on the ﬂoor, R-

Studio is like an elegant picture frame. R hangs in the middle of R

studio, and like any good picture frame, enhances our appreciation

of what is inside it.

The website for R-studio is http://www.rstudio.org/ and you can

inspect the information there at any time. For most of the rest of

this chapter, if you want to follow along with the installation and

use of R-Studio, you will need to work on a Mac, Windows, or

Linux computer.

Before we start that, let’s consider why we need an IDE to work

with R. In the previous chapters, we have typed a variety of com-

mands into R, using what is known as the "R console." Console is

an old technology term that dates back to the days when comput-

ers were so big that they each occupied their own air conditioned

room. Within that room there was often one "master control sta-

tion" where a computer operator could do just about anything to

control the giant computer by typing in commands. That station

was known as the console. The term console is now used in many

cases to refer to any interface where you can directly type in com-

mands. We’ve typed commands into the R console in an effort to

learn about the R language as well as to illustrate some basic princi-

ples about data structures and statistics.

If we really want to "do" data science, though, we can’t sit around

typing commands every day. First of all, it will become boring very

fast. Second of all, whoever is paying us to be a data scientist will

get suspicious when he or she notices that we are retyping some of

the commands we typed yesterday. Third, and perhaps most impor-

tant, it is way too easy to make a mistake - to create what computer

scientists refer to as a bug - if you are doing every little task by

hand. For these reasons, one of our big goals within this book is to

create something that is reusable: where we can do a few clicks or

type a couple of things and unleash the power of many processing

steps. Using an IDE, we can build these kinds of reusable pieces.

The IDE gives us the capability to open up the process of creation,

to peer into the component parts when we need to, and to close the

hood and hide them when we don’t. Because we are working with

data, we also need a way of closely inspecting the data, both its con-

tents and its structure. As you probably noticed, it gets pretty tedi-

ous doing this at the R console, where almost every piece of output

is a chunk of text and longer chunks scroll off the screen before you

can see them. As an IDE for R, R-Studio allows us to control and

66

monitor both our code and our text in a way that supports the crea-

tion of reusable elements.

Before we can get there, though, we have to have R-Studio in-

stalled on a computer. Perhaps the most challenging aspect of in-

stalling R-Studio is having to install R ﬁrst, but if you’ve already

done that in chapter 2, then R-Studio should be a piece of cake.

Make sure that you have the latest version of R installed before

you begin with the installation of R-studio. There is ample docu-

mentation on the R-studio website,

http://www.rstudio.org/, so if you follow the instructions

there, you should have minimal difﬁculty. If you reach a

page where you are asked to choose between installing R-

studio server and installing R-studio as a desktop applica-

tion on your computer, choose the latter. We will look into R-

studio server a little later, but for now you want the

desktop/single user version. If you run into any difﬁculties

or you just want some additional guidance about R-studio,

you may want to have a look at the book entitled, Getting

Started with R-studio, by John Verzani (2011, Sebastopol, CA:

O’Reilly Media). The ﬁrst chapter of that book has a general

orientation to R and R-studio as well as a guide to installing

and updating R-studio. There is also a YouTube video that

introduces R-studio here: !

http://www.youtube.com/watch?v=7sAmqkZ3Be8 !

Be aware if you search for other YouTube videos that there is

a disk recovery program as well a music group that share the

R-Studio name: You will get a number of these videos if you

search on "R-Studio" without any other search terms.

Once you have installed R-Studio, you can run it immedi-

ately in order to get started with the activities in the later parts of

this chapter. Unlike other introductory materials, we will not walk

through all of the different elements of the R-Studio screen. Rather,

as we need each feature we will highlight the new aspect of the ap-

plication. When you run R-Studio, you will see three or four sub-

windows. Use the File menu to click "New" and in the sub-menu

for "New" click "R Script." This should give you a screen that looks

something like this:

67

The upper left hand "pane" (another name for a sub-window) dis-

plays a blank space under the tab title "Untitled1." Click in that

pane and type the following:

MyMode <- function(myVector)

{

return(myVector)

}

You have just created your ﬁrst "function" in R. A function is a bun-

dle of R code that can be used over and over again without having

to retype it. Other programming languages also have functions.

Other words for function are "procedure" and "subroutine," al-

though these terms can have a slightly different meaning in other

languages. We have called our function "MyMode." You may re-

member from a couple of chapters that the basic setup of R does

not have a statistical mode function in it, even though it does have

functions for the two other other common central tendency statis-

tics, mean() and median(). We’re going to ﬁx that problem by creat-

ing our own mode function. Recall that the mode function should

count up how many of each value is in a list and then return the

value that occurs most frequently. That is the deﬁnition of the statis-

tical mode: the most frequently occurring item in a vector of num-

bers.

A couple of other things to note: The ﬁrst is the "myVector" in pa-

rentheses on the ﬁrst line of our function. This is the "argument" or

input to the function. We have seen arguments before when we

called functions like mean() and median(). Next, note the curly

braces that are used on the second and ﬁnal lines. These curly

braces hold together all of the code that goes in our function. Fi-

nally, look at the return() right near the end of our function. This

return() is where we send back the result of what our function ac-

complished. Later on when we "call" our new function from the R

console, the result that we get back will be whatever is in the paren-

theses in the return().

Based on that explanation, can you ﬁgure out what MyMode()

does in this primitive initial form? All it does is return whatever

we give it in myVector, completely unchanged. By the way, this is a

common way to write code, by building up bit by bit. We can test

out what we have each step of the way. Let’s test out what we have

accomplished so far. First, let’s make a very small vector of data to

work with. In the lower left hand pane of R-studio you will notice

that we have a regular R console running. You can type commands

into this console, just like we did in previous chapters just using R:

> tinyData <- c(1,2,1,2,3,3,3,4,5,4,5)

> tinyData

[1] 1 2 1 2 3 3 3 4 5 4 5

Then we can try out our new MyMode() function:

> MyMode(tinyData)

Error: could not find function "MyMode"

Oops! R doesn’t know about our new function yet. We typed our

MyMode() function into the code window, but we didn’t tell R

about it. If you look in the upper left pane, you will see the code

for MyMode() and just above that a few small buttons on a tool

bar. One of the buttons looks like a little right pointing arrow with

68

the word "Run" next to it. First, use your mouse to select all of the

code for MyMode(), from the ﬁrst M all the way to the last curly

brace. Then click the Run button. You will immediately see the

same code appear in the R console window just below. If you have

typed everything correctly, there should be no errors or warnings.

Now R knows about our MyMode() function and is ready to use it.

Now we can type:

> MyMode(tinyData)

[1] 1 2 1 2 3 3 3 4 5 4 5

This did exactly what we expected: it just echoed back the contents

of tinyData. You can see from this example how parameters work,

too. in the command just above, we passed in tinyData as the input

to the function. While the function was working, it took what was

in tinyData and copied it into myVector for use inside the function.

Now we are ready to add the next command to our function:

MyMode <- function(myVector)

{

uniqueValues <- unique(myVector)

return(uniqueValues)

}

Because we made a few changes, the whole function appears again

above. Later, when the code gets a little more complicated, we will

just provide one or two lines to add. Let’s see what this code does.

First, don’t forget to select the code and click on the Run button.

Then, in the R console, try the MyMode() command again:

> MyMode(tinyData)

[1] 1 2 3 4 5

Pretty easy to see what the new code does, right? We called the

unique() function, and that returned a list of unique values that ap-

peared in tinyData. Basically, unique() took out all of the redundan-

cies in the vector that we passed to it. Now let’s build a little more:

MyMode <- function(myVector)

{

uniqueValues <- unique(myVector)

uniqueCounts <- tabulate(myVector)

return(uniqueCounts)

}

Don’t forget to select all of this code and Run it before testing it

out. This time when we pass tinyData to our function we get back

another list of ﬁve elements, but this time it is the count of how

many times each value occurred:

> MyMode(tinyData)

[1] 2 2 3 2 2

Now we’re basically ready to ﬁnish our MyMode() function, but

let’s make sure we understand the two pieces of data we have in

uniqueValues and uniqueCounts:

In the table below we have lined up a row of the elements of

uniqueValues just above a row of the counts of how many of each

of those values we have. Just for illustration purposes, in the top/

69

label row we have also shown the "index" number. This index num-

ber is the way that we can "address" the elements in either of the

variables that are shown in the rows. For instance, element number

4 (index 4) for uniqueValues contains the number four, whereas ele-

ment number four for uniqueCounts contains the number two. So

if we’re looking for the most frequently occurring item, we should

look along the bottom row for the largest number. When we get

there, we should look at the index of that cell. Whatever that index

is, if we look in the same cell in uniqueValues, we will have the

value that occurs most frequently in the original list. In R, it is easy

to accomplish what was described in the last sentence with a single

line of code:

uniqueValues[which.max(uniqueCounts)]

The which.max() function ﬁnds the index of the element of unique-

Counts that is the largest. Then we use that index to address

uniqueValues with square braces. The square braces let us get at

any of the elements of a vector. For example, if we asked for

uniqueValues[5] we would get the number 5. If we add this one list

of code to our return statement, our function will be ﬁnished:

MyMode <- function(myVector)

{

uniqueValues <- unique(myVector)

uniqueCounts <- tabulate(myVector)

return(uniqueValues[which.max(uniqueCounts)])

}

We’re now ready to test out our function. Don’t forget to select the

whole thing and run it! Otherwise R will still be remembering our

old one. Let’s ask R what tinyData contains, just to remind our-

selves, and then we will send tinyData to our MyMode() function:

> tinyData

[1] 1 2 1 2 3 3 3 4 5 4 5

> MyMode(tinyData)

[1] 3

Hooray! It works. Three is the most frequently occurring value in

tinyData. Let’s keep testing and see what happens:

> tinyData<-c(tinyData,5,5,5)

> tinyData

[1] 1 2 1 2 3 3 3 4 5 4 5 5 5 5

> MyMode(tinyData)

[1] 5

70

INDEX

1

2

3

4

5

uniqueValues

1

2

3

4

5

uniqueCounts

2

2

3

2

2

It still works! We added three more ﬁves to the end of the tinyData

vector. Now tinyData contains ﬁve ﬁves. MyMode() properly re-

ports the mode as ﬁve. Hmm, now let’s try to break it:

> tinyData

[1] 1 2 1 2 3 3 3 4 5 4 5 5 5 5 1 1 1

> MyMode(tinyData)

[1] 1

This is interesting: Now tinyData contains ﬁve ones and ﬁve ﬁves.

MyMode() now reports the mode as one. This turns out to be no

surprise. In the documentation for which.max() it says that this

function will return the ﬁrst maximum it ﬁnds. So this behavior is

to be expected. Actually, this is always a problem with the statisti-

cal mode: there can be more than one mode in a data set. Our My-

Mode() function is not smart enough to realize this, not does it give

us any kind of warning that there are multiple modes in our data.

It just reports the ﬁrst mode that it ﬁnds.

Here’s another problem:

> tinyData<-c(tinyData,9,9,9,9,9,9,9)

> MyMode(tinyData)

[1] NA

> tabulate(tinyData)

[1] 5 2 3 2 5 0 0 0 7

In the ﬁrst line, we stuck a bunch of nines on the end of tinyData.

Remember that we had no sixes, sevens, or eights. Now when we

run MyMode() it says "NA," which is R’s way of saying that some-

thing went wrong and you are getting back an empty value. It is

probably not obvious why things went whacky until we look at the

last command above, tabulate(tinyData). Here we can see what

happened: when it was run inside of the MyMode() function, tabu-

late() generated a longer list than we were expecting, because it

added zeroes to cover the sixes, sevens, and eights that were not

there. The maximum value, out at the end is 7, and this refers to

the number of nines in tinyData. But look at what the unique()

function produces:

> unique(tinyData)

[1] 1 2 3 4 5 9

There are only six elements in this list, so it doesn’t match up as it

should (take another look at the table on the previous page and

imagine if the bottom row stuck out further than the row just

above it). We can ﬁx this with the addition of the match() function

to our code:

MyMode <- function(myVector)

{

uniqueValues <- unique(myVector)

uniqueCounts <- tabulate( + "

match(myVector,uniqueValues))

return(uniqueValues[which.max(uniqueCounts)])

}

71

The new part of the code is in bold. Now instead of tabulating

every possible value, including the ones for which we have no

data, we only tabulate those items where there is a "match" be-

tween the list of unique values and what is in myVector. Now

when we ask MyMode() for the mode of tinyData we get the cor-

rect result:

> MyMode(tinyData)

[1] 9

Aha, now it works the way it should. After our last addi-

tion of seven nines to the data set, the mode of this vector is

correctly reported as nine.

Before we leave this activity, make sure to save your work.

Click anywhere in the code window and then click on the

File menu and then on Save. You will be prompted to

choose a location and provide a ﬁlename. You can call the

ﬁle MyMode, if you like. Note that R adds the "R" extension

to the ﬁlename so that it is saved as MyMode.R. You can

open this ﬁle at any time and rerun the MyMode() function

in order to deﬁne the function in your current working ver-

sion of R.

A couple of other points deserve attention. First, notice that

when we created our own function, we had to do some test-

ing and repairs to make sure it ran the way we wanted it to.

This is a common situation when working on anything re-

lated to computers, including spreadsheets, macros, and pretty

much anything else that requires precision and accuracy. Second,

we introduced at least four new functions in this exercise, includ-

ing unique(), tabulate(), match(), and which.max(). Where did

these come from and how did we know? R has so many functions

that it is very difﬁcult to memorize them all. There’s almost always

more than one way to do something, as well. So it can be quite con-

fusing to create a new function, if you don’t know all of the ingredi-

ents and there’s no one way to solve a particular problem. This is

where the community comes in. Search online and you will ﬁnd

dozens of instances where people have tried to solve similar prob-

lems to the one you are solving, and you will also ﬁnd that they

have posted the R code for their solutions. These code fragments

are free to borrow and test. In fact, learning from other people’s ex-

72

amples is a great way to expand your horizons and learn new tech-

niques.

The last point leads into the next key topic. We had to do quite a

bit of work to create our MyMode function, and we are still not

sure that it works perfectly on every variation of data it might en-

counter. Maybe someone else has already solved the same prob-

lem. If they did, we might be able to ﬁnd an existing "package" to

add onto our copy of R to extend its functions. In fact, for the statis-

tical mode, there is an existing package that does just about every-

thing you could imagine doing with the mode. The package is

called modeest, a not very good abbreviation for mode-estimator.

To install this package look in the lower right hand pane of R-

studio. There are several tabs there, and one of them is "Packages."

Click on this and you will get a list of every package that you al-

ready have available in your copy of R (it may be a short list) with

checkmarks for the ones that are ready to use. It is unlikely that

modeest is already on this list, so click on the button that says "In-

stall Packages. This will give a dialog that looks like what you see

on the screenshot above. Type the beginning of the package name

in the appropriate area, and R-studio will start to prompt you with

matching choices. Finish typing modeest or choose it off of the list.

There may be a check box for "Install Dependencies," and if so

leave this checked. In some cases an R package will depend on

other packages and R will install all of the necessary packages in

the correct order if it can. Once you click the "Install" button in this

dialog, you will see some commands running on the R console (the

lower left pane). Generally, this works without a hitch and you

should not see any warning messages. Once the installation is com-

plete you will see modeest added to the list in the lower right pane

(assuming you have clicked the "Packages" tab). One last step is to

click the check box next to it. This runs the library() function on the

package, which prepares it for further use.

Let’s try out the mfv() function. This function returns the "most fre-

quent value" in a vector, which is generally what we want in a

mode function:

> mfv(tinyData)

[1] 9

So far so good! This seems to do exactly what our MyMode() func-

tion did, though it probably uses a different method. In fact, it is

easy to see what strategy the authors of this package used just by

typing the name of the function at the R command line:

> mfv

function (x, ...)

{

f <- factor(x)

tf <- tabulate(f)

return(as.numeric(levels(f)[tf == max(tf)]))

}

<environment: namespace:modeest>

This is one of the great things about an open source program: you

can easily look under the hood to see how things work. Notice that

this is quite different from how we built MyMode(), although it too

uses the tabulate() function. The ﬁnal line, that begins with the

word "environment" has importance for more complex feats of pro-

73

gramming, as it indicates which variable names mfv() can refer to

when it is working. The other aspect of this function which is

probably not so obvious is that it will correctly return a list of multi-

ple modes when one exists in the data you send to it:

> multiData <- c(1,5,7,7,9,9,10)

> mfv(multiData)

[1] 7 9

> MyMode(multiData)

[1] 7

In the ﬁrst command line above, we made a small new vector that

contains two modes, 7 and 9. Each of these numbers occurs twice,

while the other numbers occur only once. When we run mfv() on

this vector it correctly reports both 7 and 9 as modes. When we use

our function, MyMode(), it only reports the ﬁrst of the two modes.

To recap, this chapter provided a basic introduction to R-studio, an

integrated development environment (IDE) for R. An IDE is useful

for helping to build reusable components for handling data and

conducting data analysis. From this point forward, we will use R-

studio, rather than plain old R, in order to save and be able to re-

use our work. Among other things, R-studio makes it easy to man-

age "packages" in R, and packages are the key to R’s extensibility.

In future chapters we will be routinely using R packages to get ac-

cess to specialized capabilities.

These specialized capabilities come in the form of extra functions

that are created by developers in the R community. By creating our

own function, we learn that functions take "arguments" as their in-

puts and provide a return value. A return value is a data object, so

it could be a single number (technically a vector of length one) or it

could be a list of values (a vector) or even a more complex data ob-

ject. We can write and reuse our own functions, which we will do

quite frequently later in the book, or we can use other people’s

functions by installing their packages and using the library() func-

tion to make the contents of the package available. Once we have

used library() we can inspect how a function works by typing its

name at the R command line. (Note that this works for many func-

tions, but there are a few that were created in a different computer

language, like C, and for those we will not be able to inspect the

code as easily.)

Chapter Challenge

Write and test a new function called MySamplingDistribution()

that creates a sampling distribution of means from a numeric input

vector. You will need to integrate your knowledge of creating new

functions from this chapter with your knowledge of creating sam-

pling distributions from the previous chapter in order to create a

working function. Make sure to give careful thought about the pa-

rameters you will need to pass to your function and what kind of

data object your function will return.

Sources

http://en.wikipedia.org/wiki/R_(programming_language)

http://en.wikipedia.org/wiki/Joseph_J._Allaire

http://stats.lse.ac.uk/penzer/ST419materials/CSchpt3.pdf

74

http://www.use-r.org/downloads/Getting_Started_with_RStudio

.pdf

http://www.statmethods.net/interface/packages.html

http://www.youtube.com/watch?v=7sAmqkZ3Be8

R Commands Used in this Chapter

function() - Creates a new function

return() - Completes a function by returning a value

tabulate() - Counts occurrences of integer-valued data in a vector

unique() - Creates a list of unique values in a vector

match() - Takes two lists and returns values that are in each

mfv() - Most frequent value (from the modeest package)

75

Review 9.1 Onward with R-Studio

Check Answer

Question 1 of 5

One common deﬁnition for the statistical mode is:

A.

The sum of all values divided by the

number of values.

B.

The most frequently occurring value

in the data.

C.

The halfway point through the data.

D. The distance between the smallest

value and the largest value.

We’ve come a long way already: Basic skills in controlling R, some exposure to R-studio, knowledge

of how to manage add-on packages, experience creating a function, essential descriptive statistics, and

a start on sampling distributions and inferential statistics. In this chapter, we use the social media

service Twitter to grab some up-to-the minute data and begin manipulating it.

CHAPTER 10

76

Tweet, Tweet!

Prior to this chapter we only worked with toy data sets: some

made up data about a ﬁctional family and the census head

counts for the 50 states plus the District of Columbia. At this

point we have practiced a sufﬁcient range of skills to work

with some real data. There are data sets everywhere, thou-

sands of them, many free for the taking, covering a range of

interesting topics from psychology experiments to ﬁlm actors.

For sheer immediacy, though, you can’t beat the Twitter so-

cial media service. As you may know from direct experience,

Twitter is a micro-blogging service that allows people all over

the world to broadcast brief thoughts (140 characters or less)

that can then be read by their "followers" (other Twitter users

who signed up to receive the sender’s messages). The devel-

opers of Twitter, in a stroke of genius, decided to make these

postings, called tweets, available to the general public

through a web page on the Twitter.com site, and additional

through what is known as an application programming inter-

face or API.

Here’s where the natural extensibility of R comes in. An indi-

vidual named Jeff Gentry who, at this writing, seems to be a

data professional in the ﬁnancial services industry, created an

add-on package for R called twitteR (not sure how it is pro-

nounced, but "twit-are" seems pretty close). The twitteR pack-

age provides an extremely simple interface for downloading

a list of tweets directly from the Twitter service into R. Using

the interface functions in twitteR, it is possible to search

through Twitter to obtain a list of tweets on a speciﬁc topic.

Every tweet contains the text of the posting that the author

wrote as well as lots of other useful information such as the

time of day when a tweet was posted. Put it all together and

it makes a fun way of getting up-to-the-minute data on what

people are thinking about a wide variety of topics.

The other great thing about working with twitteR is that we

will use many, if not all of the skills that we have developed

earlier in the book to put the interface to use.

A Token of Your Esteem: Using OAuth

Before we move forward with creating some code in R-

studio, there’s an important set of steps we need to accom-

plish at the Twitter website.

In 2013, Twitter completed a transition to a new version of

their application programming interface, or API. This new

API requires the use of a technique for authorization - a way

of proving to Twitter that you are who you are when you

search for (or post) tweets from a software application. The

folks at Twitter adopted an industry standard for this process

known as OAuth. OAuth provides a method for obtaining

two pieces of information - a "secret" and a "key" - without

which it will be difﬁcult if not downright impossible to work

with Twitter (as well as twitteR). Here are the steps:

1.!Get a Twitter account at Twitter.com if you don’t already

have one.

2.!Go to the development page at Twitter

(https://dev.twitter.com) and sign in with your Twitter cre-

dentials.

77

3.!Click on "My Applications." The location of this may vary

over time, but look for in a drop down list that is under

your proﬁle picture on the top right corner of the screen.

4.!Click on "Create a New Application." Fill in the blanks

with some sensible answers. Where it asks for a “website”

you can give your own home page. This is a required re-

sponse, so you will have to have some kind of web page to

point to. In contrast, the “Callback URL” can be left blank.

Click submit.

5.!Check the checkbox speciﬁed in the image below under set-

tings. Your application should be set so that it can be used

to sign in with Twitter.

6.!You will get a screen containing a whole bunch of data.

Make sure to save it all, but the part that you will really

need is the "Consumer key" and the "Consumer Secret,"

both of which are long strings of letters and numbers.

These strings will be used later to get your application run-

ning in R. The reason these are such long strings of gibber-

ish is that they are encrypted.

7.!Also take note of the Request Token URL and the Author-

ize URL. For the most part these are exactly the same

across all uses of Twitter, but they may change over time,

so you should make sure to stash them away for later. You

do not need to click on the “Create my Access Token” but-

ton.

8.!Go to the Settings tab and make sure that "Read, Write and

Access direct messages" is set.

You may notice on the Home->My applications screen in the

dev.twitter.com interface that there are additional tabs along

the top for different activities and tasks related to OAuth.

There is a tab called "OAuth tool" where you can always

come back to get your Consumer key and Consumer secret

information. Later in the chapter we will come back to the us-

age of your Consumer key and your Consumer secret but be-

fore we get there we have to get the twitteR package ready to

go.

Working with twitteR

To begin working with twitteR, launch your copy of R-studio.

The ﬁrst order of business is to create a new R-studio "pro-

ject". A project in R-studio helps to keep all of the different

pieces and parts of an activity together including the datasets

and variables that you establish as well as the functions that

you write. For professional uses of R and R-studio, it is impor-

tant to have one project for each major activity: this keeps dif-

ferent data sets and variable names from interfering with

each other. Click on the "Project" menu in R-studio and then

click on "New Project." You will usually have a choice of three

kinds of new projects, a brand new "clean" project, an existing

directory of ﬁles that will get turned into a project folder, or a

project that comes out of a version control system. (Later in

the book we will look at version control, which is great for

projects involving more than one person.) Choose "New Di-

rectory" to start a brand new project. You can call your project

78

whatever you want, but because this project uses the twitteR

package, you might want to just call the project "twitter". You

also have a choice in the dialog box about where on your com-

puter R-studio will create the new directory.

R-studio will respond by showing a clean console screen and

most importantly an R "workspace" that does not contain any

of the old variables and data that we created in previous chap-

ters. In order to use twitteR, we need to load several packages

that it depends upon. These are called, in order "bitops",

"RCurl", "RJSONIO", and once these are all in place "twitteR"

itself. Rather than doing all of this by hand with the menus,

let’s create some functions that will assist us and make the ac-

tivity more repeatable. First, here is a function that takes as

input the name of a package. It tests whether the package has

been downloaded - "installed" - from the R code repository. If

it has not yet been downloaded/installed, the function takes

care of this. Then we use a new function, called require(), to

prepare the package for further use. Let’s call our function

"EnsurePackage" because it ensures that a package is ready

for us to use. If you don’t recall this step from the previous

chapter, you should click the "File" menu and then click

"New" to create a new ﬁle of R script. Then, type or copy/

paste the following code:

EnsurePackage<-function(x)

{

x <- as.character(x)

if (!require(x,character.only=TRUE))

{

install.packages(pkgs=x,"

repos="http://cran.r-project.org")

require(x,character.only=TRUE)

}

}

On Windows machines, the folder where new R packages are

stored has to be conﬁgured to allow R to put new ﬁles there

(“write” permissions). In Windows Explorer, you can right

click on the folder and choose “Properties->Security” then

choose your username and user group, click Edit, enable all

permissions, and click OK. If you run into trouble, check out

the Windows FAQ at CRAN by searching or using this web

address:

http://cran.r-project.org/bin/windows/base/rw-FAQ.html .

The require() function on the fourth line above does the same

thing as library(), which we learned in the previous chapter,

but it also returns the value "FALSE" if the package you re-

quested in the argument "x" has not yet been downloaded.

That same line of code also contains another new feature, the

"if" statement. This is what computer scientists call a condi-

tional. It tests the stuff inside the parentheses to see if it evalu-

ates to TRUE or FALSE. If TRUE, the program continues to

run the script in between the curly braces (lines 4 and 8). If

FALSE, all the stuff in the curly braces is skipped. Also in the

third line, in case you are curious, the arguments to the re-

quire() function include "x," which is the name of the package

that was passed into the function, and "character.only=TRUE"

which tells the require() function to expect x to be a character

79

string. Last thing to notice about this third line: there is a "!"

character that reverses the results of the logical test. Techni-

cally, it is the Boolean function NOT. It requires a bit of men-

tal gyration that when require() returns FALSE, the "!" inverts

it to TRUE, and that is when the code in the curly braces

runs.

Once you have this code in a script window, make sure to se-

lect the whole function and click Run in the toolbar to make R

aware of the function. There is also a checkbox on that same

toolbar called, "Source on Save," that will keep us from hav-

ing to click on the Run button all the time. If you click the

checkmark, then every time you save the source code ﬁle, R-

studio will rerun the code. If you get in the habit of saving af-

ter every code change you will always be running the latest

version of your function.

Now we are ready to put EnsurePackage() to work on the

packages we need for twitteR. We’ll make a new function,

"PrepareTwitter," that will load up all of our packages for us.

Here’s the code:

PrepareTwitter<-function()

{

EnsurePackage("bitops")

EnsurePackage("RCurl")

EnsurePackage("RJSONIO")

EnsurePackage("twitteR")

EnsurePackage("ROAuth")

}

This code is quite straightforward: it calls the EnsurePack-

age() function we created before ﬁve times, once to load each

of the packages we need. You may get some warning mes-

sages and these generally won’t cause any harm. If you are

on Windows and you get errors about being able to write to

your library remember to check the Windows FAQ as noted

above.

Make sure to save your script ﬁle once you have typed this

new function in. You can give it any ﬁle name that make

sense to you, such as "twitterSupport." Now is also a good

time to start the habit of commenting: Comments are human

readable messages that software developers leave for them-

selves and for others, so that everyone can remember what a

piece of code is supposed to do. All computer languages have

at least one "comment character" that sets off the human read-

able stuff from the rest of the code. In R, the comment charac-

ter is #. For now, just put one comment line above each func-

tion you created, brieﬂy describing it, like this:

# EnsurePackage(x) - Installs and loads a package

# if necessary

and this:

# PrepareTwitter() - Load packages for working

# with twitteR

Later on we will do a better job a commenting, but this gives

us the bare minimum we need to keep going with this pro-

ject. Before we move on, you should run the PrepareTwitter()

80

function on the console command line to actually load the

packages we need:

> PrepareTwitter()

Note the parentheses after the function name, even though

there is no argument to this function. What would happen if

you left out the parentheses? Try it later to remind yourself of

some basic R syntax rules.

You may get a lot of output from running PrepareTwitter(),

because your computer may need to download some or all of

these packages. You may notice the warning message above,

for example,, about objects being "masked." Generally speak-

ing, this message refers to a variable or function that has be-

come invisible because another variable or function with the

same name has been loaded. Usually this is ﬁne: the newer

thing works the same as the older thing with the same name.

Take a look at the four panes in R-Studio, each of which con-

tains something of interest. The upper left pane is the code/

script window, where we should have the code for our two

new functions. The lower left pane shows our R console with

the results of the most recently run commands. The upper

right pane contains our workspace and history of prior com-

mands, with the tab currently set to workspace. As a re-

minder, in R parlance, workspace represents all of the cur-

rently available data objects include functions. Our two new

functions which we have deﬁned should be listed there, indi-

cating that they have each run at least once and R is now

aware of them. In the lower right pane, we have ﬁles, plots,

packages, and help, with the tab currently set to packages.

This window is scrolled to the bottom to show that RCurl,

RJSONIO, and twitteR are all loaded and "libraryed" meaning

that they are ready to use from the command line or from

functions.

Getting New SSL Tokens on Windows

For Windows users, depending upon which version of operat-

ing system software you are using as well as your upgrade

history, it may be necessary to provide new SSL certiﬁcates.

Certiﬁcates help to maintain secure communications across

the Internet, and most computers keep an up-to-date copy on

ﬁle, but not all of them do. If you encounter any problems us-

ing R to access the Internet, you may need new tokens.

download.file(url="http://curl.haxx.se/ca/cacert.pem",+

destfile="cacert.pem")

This statement needs to be run before the R tries to contact

Twitter for authentication. This is because twitteR uses RCurl

which in turn employs SSL security whenever “https” ap-

pears in a URL. The command above downloads new certiﬁ-

cates and saves them within the current working directory

for R. You may need to use cacert.pem for many or most of

the function calls to twitteR by adding the argument

cainfo="cacert.pem".

Using Your OAuth Tokens

Remember at the beginning of the chapter that we went

through some rigamarole to get a Consumer key and a Con-

sumer secret from Twitter. Before we can get started in retriev-

81

ing data from Twitter we need to put those long strings of

numbers and letters to use.

Begin this process by getting a credential from ROAuth. Re-

member that in the command below where I have put "letter-

sAndNumbers" you have to substitute in your ConsumerKey

and your ConsumerSecret that you got from Twitter. The Con-

sumerKey is a string of upper and lowercase letters and dig-

its about 22 characters long. The ConsumerSecret is also let-

ters and digits and it is about twice as long as the Con-

sumerKey. Make sure to keep these private, especially the

ConsumerSecret, and don’t share them with others. Here’s

the command:

> credential <-

OAuthFactory$new(consumerKey="lettersAndNumbers","

+cons

umerSecret="lettersAndNumbers","

+requestURL="https://api.twitter.com/oauth/request_t

oken","

+accessURL="https://api.twitter.com/oauth/access_tok

en","

+authURL="https://api.twitter.com/oauth/authorize")

This looks messy but is really very simple. If you now type:

> credential

You will ﬁnd that the credential data object is just a conglom-

eration of the various ﬁelds that you speciﬁed in the argu-

ments to the OAuthFactory$new method. We have to put that

data structure to work now with the following function call:

> credential$handshake()

Or, on Windows machines, if you have downloaded new cer-

tiﬁcates:

> credential$handshake(cainfo="cacert.pem")

You will get a response back that looks like this:

When complete, record the PIN given to you and provide it

here:

To enable the connection, please direct your web browser to:

https://api.twitter.com/oauth/authorize?oauth_token=...

When complete, record the PIN given to you and provide it here:

This will be followed by a long string of numbers. Weirdly,

you have to go to a web browser and type in exactly what

you see in the R-Studio output window (the URL and the

long string of numbers). While typing the URL to be redi-

rected to twitter, be sure that you type http:// instead of

https:// otherwise Twitter will not entertain the request be-

cause the Twitter server invokes SSL security itself. If you

type the URL correctly, Twitter will respond in your browser

window with a big button that says "Authorize App." Go

ahead and click on that and you will receive a new screen

with a PIN on it (my PIN had seven digits). Take those seven

digits and type them into the R-Studio console window (the

credential$handshake() function will be waiting for them).

Type the digits in front of “When complete, record the PIN

given to you and provide it here:” Hit Enter and, assuming

you get no errors, you are fully authorized! Hooray! What a

crazy process! Thankfully, you should not have to do any of

82

this again as long as you save the credential data object and

restore it into future sessions. The credential object, and all of

the other active data, will be stored in the default workspace

when you exit R or R-Studio. Make sure you know which

workspace it was saved in so you can get it back later.

Ready, Set, Go!

Now let’s get some data from Twitter. First, tell the twitteR

package that you want to use your shiny new credentials:

> registerTwitterOAuth(credential)

[1] TRUE

The return value of TRUE shows that the credential is work-

ing and ready to help you get data from Twitter. Subsequent

commands using the twitteR package will pass through the

authorized application interface.

The twitteR package provides a function called searchTwit-

ter() that allows us to retrieve some recent tweets based on a

search term. Twitter users have invented a scheme for organ-

izing their tweets based on subject matter. This system is

called "hashtags" and is based on the use of the hashmark

character (#) followed by a brief text tag. For example, fans of

Oprah Winfrey use the tag #oprah to identify their tweets

about her. We will use the searchTwitter() function to search

for hashtags about global climate change. The website

hashtags.org lists a variety of hashtags covering a range of

contemporary topics. You can pick any hashtag you like, as

long as there are a reasonable number of tweets that can be

retrieved. The searchTwitter() function also requires specify-

ing the maximum number of tweets that the call will return.

For now we will use 500, although you may ﬁnd that your re-

quest does not return that many. Here’s the command:

tweetList <- searchTwitter("#climate", n=500)

As above, if you are on Windows, and you had to get new cer-

tiﬁcates, you may have to use this command:

tweetList <- searchTwitter("#climate", n=500,

cainfo="cacert.pem")

Depending upon the speed of your Internet connection and

the amount of trafﬁc on Twitter’s servers, this command may

take a short while for R to process. Now we have a new data

object, tweetList, that presumably contains the tweets we re-

quested. But what is this data object? Let’s use our R diagnos-

tics to explore what we have gotten:

> mode(tweetList)

[1] "list"

Hmm, this is a type of object that we have not encountered

before. In R, a list is an object that contains other data objects,

and those objects may be a variety of different modes/types.

Contrast this deﬁnition with a vector: A vector is also a kind

of list, but with the requirement that all of the elements in the

vector must be in the same mode/type. Actually, if you dig

deeply into the deﬁnitions of R data objects, you may realize

that we have already encountered one type of list: the data-

frame. Remember that the dataframe is a list of vectors,

where each vector is exactly the same length. So a dataframe

is a particular kind of list, but in general lists do not have

83

those two restrictions that dataframes have (i.e., that each ele-

ment is a vector and that each vector is the same length).

So we know that tweetList is a list, but what does that list con-

tain? Let’s try using the str() function to uncover the structure

of the list:

str(tweetList)

Whoa! That output scrolled right off the screen. A quick

glance shows that it is pretty repetitive, with each 20 line

block being quite similar. So let’s use the head() function to

just examine the ﬁrst element of the list. The head() function

allows you to just look at the ﬁrst few elements of a data ob-

ject. In this case we will look just at the ﬁrst list element of the

tweetList list. The command, also shown on the screen shot

below is:

str(head(tweetList,1))

Looks pretty messy, but is simpler than it may ﬁrst appear.

Following the line "List of 1," there is a line that begins "$ :Ref-

erence class" and then the word ‘status’ in single quotes. In

Twitter terminology a "status" is a single tweet posting (it sup-

posedly tells us the "status" of the person who posted it). So

the author of the R twitteR package has created a new kind of

data object, called a ‘status’ that itself contains 10 ﬁelds. The

ﬁelds are then listed out. For each line that begins with "..$"

there is a ﬁeld name and then a mode or data type and then a

taste of the data that that ﬁeld contains.

So, for example, the ﬁrst ﬁeld, called "text" is of type "chr"

(which means character/text data) and the ﬁeld contains the

string that starts with, "Get the real facts on gas prices." You

can look through the other ﬁelds and see if you can make

sense of them. There are two other data types in there: "logi"

stands for logical and that is the same as TRUE/FALSE; "PO-

SIXct" is a format for storing the calendar date and time. (If

you’re curious, POSIX is an old unix style operating system,

where the current date and time were stored as the number of

seconds elapsed since 12 midnight on January 1, 1970.) You

can see in the "created" ﬁeld that this particular tweet was cre-

ated on April 5, 2012 one second after 2:10 PM. It does not

84

show what time zone, but a little detective work shows that

all Twitter postings are coded with "coordinated universal

time" or what is usually abbreviated with UTC.

One last thing to peek at in this data structure is about seven

lines from the end, where it says, "and 33 methods..." In com-

puter science lingo a "method" is an operation/activity/

procedure that works on a particular data object. The idea of

a method is at the heart of so called "object oriented program-

ming." One way to think of it is that the data object is the

noun, and the methods are all of the verbs that work with

that noun. For example you can see the method "getCreated"

in the list: If you use the method getCreated on an reference

object of class ‘status’, the method will return the creation

time of the tweet.

If you try running the command:

str(head(tweetList,2))

you will ﬁnd that the second item in the tweetList list is struc-

tured exactly like the ﬁrst time, with the only difference being

the speciﬁc contents of the ﬁelds. You can also run:

length(tweetList)

to ﬁnd out how many items are in your list. The list obtained

for this exercise was a full 500 items long. Se we have 500

complex items in our list, but every item had exactly the same

structure, with 10 ﬁelds in it and a bunch of other stuff too.

That raises a thought: tweetList could be thought of as a 500

row structure with 10 columns! That means that we could

treat it as a dataframe if we wanted to (and we do, because

this makes handling these data much more convenient as you

found in the "Rows and Columns" chapter).

Happily, we can get some help from R in converting this list

into a dataframe. Here we will introduce four powerful new

R functions: as(), lapply(), rbind(), and do.call(). The ﬁrst of

these, as(), performs a type coercion: in other words it

changes one type to another type. The second of these, lap-

ply(), applies a function onto all of the elements of a list. In

the command below, lapply(tweetList, as.data.frame), applies

the as.data.frame() coercion to each element in tweetList.

Next, the rbind() function "binds" together the elements that

are supplied to it into a row-by-row structure. Finally, the

do.call() function executes a function call, but unlike just run-

ning the function from the console, allows for a variable num-

ber of arguments to be supplied to the function. The whole

command we will use looks like this:

tweetDF <- do.call("rbind", lapply(tweetList,

+"

as.data.frame))

You might wonder a few things about this command. One

thing that looks weird is "rbind" in double quotes. This is the

required method of supplying the name of the function to

do.call(). You might also wonder why we needed do.call() at

all. Couldn’t we have just called rbind() directly from the com-

mand line? You can try it if you want, and you will ﬁnd that

it does provide a result, but not the one you want. The differ-

ence is in how the arguments to rbind() are supplied to it: if

you call it directly, lapply() is evaluated ﬁrst, and it forms a

single list that is then supplied to rbind(). In contrast, by us-

85

ing do.call(), all 500 of the results of lapply() are supplied to

rbind() as individual arguments, and this allows rbind() to

create the nice rectangular dataset that we will need. The ad-

vantage of do.call() is that it will set up a function call with a

variable number of arguments in cases where we don’t know

how many arguments will be supplied at the time when we

write the code.

If you run the command above, you should see in the upper

right hand pane of R-studio a new entry in the workspace un-

der the heading of "Data." For the example we are running

here, the entry says, "500 obs. of 10 variables." This is just

what we wanted, a nice rectangular data set, ready to ana-

lyze. Later on, we may need more than one of these data sets,

so let’s create a function to accomplish the commands we just

ran:

# TweetFrame() - Return a dataframe based on

a"

# search of Twitter

TweetFrame<-function(searchTerm, maxTweets)

{

twtList<-

searchTwitter(searchTerm,n=maxTweets)

return(do.call("rbind",+"

lapply(twtList,as.data.frame)))

}

There are three good things about putting this code in a func-

tion. First, because we put a comment at the top of the func-

tion, we will remember in the future what this code does. Sec-

ond, if you test this function you will ﬁnd out that the vari-

able twtList that is created in the code above does not stick

around after the function is ﬁnished running. This is the re-

sult of what computer scientists call "variable scoping." The

variable twtList only exists while the TweetFrame() function

is running. Once the function is done, twtList evaporates as if

it never existed. This helps us to keep our workspace clean

and avoid collecting lots of intermediate variables that are

not reused.

The last and best thing about this function is that we no

longer have to remember the details of the method for using

do.call(), rbind(), lapply(), and as.data.frame() because we

will not have to retype these commands again: we can just

call the function whenever we need it. And we can always go

back and look at the code later. In fact, this would be a good

reason to put in a comment just above the return() function.

Something like this:

# as.data.frame() coerces each list element into a row!

# lapply() applies this to all of the elements in twtList!

# rbind() takes all of the rows and puts them together!

# do.call() gives rbind() all the rows as individual elements

Now, whenever we want to create a new data set of tweets,

we can just call TweetFrame from the R console command

line like this:

lgData <- TweetFrame("#ladygaga", 250)

86

This command would give us a new dataframe "lgData" all

ready to analyze, based on the supplied search term and maxi-

mum number of tweets.

Let’s start to play with the tweetDF dataset that we created

before. First, as a matter of convenience, let’s learn the at-

tach() function. The attach() function saves us some typing by

giving one particular dataframe priority over any others that

have the same variable names. Normally, if we wanted to ac-

cess the variables in our dataframe, we would have to use the

$ notation, like this:

tweetDF$created

But if we run attach(tweetDF) ﬁrst, we can then refer to cre-

ated directly, without having to type the tweetDF$ before it:

> attach(tweetDF)

> head(created,4)

[1] "2012-04-05 14:10:01 UTC" "2012-04-05 14:09:21

UTC"

[3] "2012-04-05 14:08:15 UTC" "2012-04-05 14:07:12

UTC"

Let’s visualize the creation time of the 500 tweets in our data-

set. When working with time codes, the hist() function re-

quires us to specify the approximate number of categories we

want to see in the histogram:

hist(created, breaks=15, freq=TRUE)

This command yields the histogram that appears below. If we

look along the x-axis (the horizontal), this string of tweets

starts at about 4:20 AM and goes until about 10:10 AM, a span

of roughly six hours. There are 22 different bars so each bar

represents about 16 minutes - for casual purposes we’ll call it

a quarter of an hour. It looks like there are something like 20

tweets per bar, so we are looking at roughly 80 tweets per

hour with the hashtag "#climate." This is obviously a pretty

popular topic. This distribution does not really have a dis-

cernible shape, although it seems like there might be a bit of a

growth trend as time goes on, particularly starting at about

7:40 AM.

87

Take note of something very important about these data: It

doesn’t make much sense to work with a measure of central

tendency. Remember a couple of chapters ago when we were

looking at the number of people who resided in different U.S.

states? In that case it made sense to say that if State A had one

million people and State B had three million people, then the

average of these two states was two million people. When

you’re working with time stamps, it doesn’t make a whole lot

of sense to say that one tweet arrived at 7 AM and another ar-

rived at 9 AM so the average is 8 AM. Fortunately, there’s a

whole area of statistics concerned with "arrival" times and

similar phenomena, dating back to a famous study by Ladis-

laus von Bortkiewicz of horsemen who died after being

kicked by their horses. von Bortkiewicz studied each of 14

cavalry corps over a period of 20 years, noting when horse-

men died each year. The distribution of the "arrival" of kick-

deaths turns out to have many similarities to other arrival

time data, such as the arrival of buses or subway cars at a sta-

tion, the arrival of customers at a cash register, or the occur-

rence of telephone calls at a particular exchange. All of these

kinds of events ﬁt what is known as a "Poisson Distribution"

(named after Simeon Denis Poisson, who published it about

half a century before von Bortkiewicz found a use for it).

Let’s ﬁnd out if the arrival times of tweets comprise a Poisson

distribution.

Right now we have the actual times when the tweets were

posted, coded as a POSIX date and time variable. Another

way to think about these data is to think of each new tweet as

arriving a certain amount of time after the previous tweet. To

ﬁgure that out, we’re going to have to "look back" a row in or-

der to subtract the creation time of the previous tweet from

the creation time of the current tweet. In order to be able to

make this calculation, we have to make sure that our data are

sorted in ascending order of arrival - in other words the earli-

est one ﬁrst and the latest one last. To accomplish this, we

will use the order() function together with R’s built-in square

bracket notation.

As mentioned brieﬂy in the previous chapter, in R, square

brackets allow "indexing" into a list, vector, or data frame. For

example, myList[3] would give us the third element of myL-

ist. Keeping in mind that a a dataframe is a rectangular struc-

ture, really a two dimensional structure, we can address any

element of a dataframe with both a row and column designa-

tor: myFrame[4,1] would give the fourth row and the ﬁrst col-

umn. A shorthand for taking the whole column of a data-

frame is to leave the row index empty: myFrame[ , 6] would

give every row in the sixth column. Likewise, a shorthand for

taking a whole row of a dataframe is to leave the column in-

dex empty: myFrame[10, ] would give every column in the

tenth row. We can also supply a list of rows instead of just

one row, like this: myFrame[ c(1,3,5), ] would return rows 1,

3, 5 (including the data for all columns, because we left the

column index blank). We can use this feature to reorder the

rows, using the order() function. We tell order() which vari-

able we want to sort on, and it will give back a list of row indi-

ces in the order we requested. Putting it all together yields

this command:

tweetDF[order(as.integer(created)), ]

88

Working our way from the inside to the outside of the expres-

sion above, we want to sort in the order that the tweets were

created. We ﬁrst coerce the variable "created" to integer - it

will then truly be expressed in the number of seconds since

1970 - just in case there are operating system differences in

how POSIX dates are sorted. We wrap this inside the order()

function. The order() function will provide a list of row indi-

ces that reﬂects the time ordering we want. We use the square

brackets notation to address the rows in tweetDF, taking all of

the columns by leaving the index after the comma empty.

We have a choice of what to do with the dataframe that is re-

turned from this command. We could assign it back to

tweetDF, which would overwrite our original dataframe with

the sorted version. Or we could create a new sorted data-

frame and leave the original data alone, like so:

sortweetDF<-tweetDF[order(as.integer(created)), ]

If you choose this method, make sure to detach() tweetDF

and attach() sortweetDF so that later commands will work

smoothly with the sorted dataframe:

> detach(tweetDF)

> attach(sortweetDF)

Another option, which seems better than creating a new data-

frame, would be to build the sorting into the TweetFrame()

function that we developed at the beginning of the chapter.

Let’s leave that to the chapter challenge. For now, we can

keep working with sortweetDF.

Technically, what we have with our created variable now is a

time series, and because statisticians like to have convenient

methods for dealing with time series, R has a built-in func-

tion, called diff(), that allows us to easily calculate the differ-

ence in seconds between each pair of neighboring values. Try

it:

diff(created)

You should get a list of time differences, in seconds, between

neighboring tweets. The list will show quite a wide range of

intervals, perhaps as long as several minutes, but with many

intervals near or at zero. You might notice that there are only

499 values and not 500: This is because you cannot calculate a

time difference for the very ﬁrst tweet, because we have no

data on the prior tweet. Let’s visualize these data and see

what we’ve got:

hist(as.integer(diff(created)))

89

As with earlier commands, we use as.integer() to coerce the

time differences into plain numbers, otherwise hist() does not

know how to handle the time differences. This histogram

shows that the majority of tweets in this group come within

50 seconds or less of the previous tweets. A much smaller

number of tweets arrive within somewhere between 50 and

100 seconds, and so on down the line. This is typical of a Pois-

son arrival time distribution. Unlike the raw arrival time

data, we could calculate a mean on the time differences:

> mean(as.integer(diff(created)))

[1] 41.12826

We have to be careful though, in using measures of central

tendency on this positively skewed distribution, that the

value we get from the mean() is a sensible representation of

central tendency. Remembering back to the previous chapter,

and our discussion of the statistical mode (the most fre-

quently occurring value), we learn that the mean and the

mode are very different:

> library("modeest")

> mfv(as.integer(diff(created)))

[1] 0

We use the library() function to make sure that the add on

package with the mfv() function is ready to use. The results of

the mfv() function show that the most commonly occurring

time interval between neighboring tweets is zero!

Likewise the median shows that half of the tweets have arri-

val times of under half a minute:

> median(as.integer(diff(created)))

[1] 28

In the next chapter we will delve more deeply into what it

means when a set of data are shaped like a Poisson distribu-

tion and what that implies about making use of the mean.

One last way of looking at these data before we close this

chapter. If we choose a time interval, such as 10 seconds, or

30 seconds, or 60 seconds, we can ask the question of how

90

many of our tweet arrivals occurred within that time interval.

Here’s code that counts the number of arrivals that occur

within certain time intervals:

> sum((as.integer(diff(created)))<60)

[1] 375

> sum((as.integer(diff(created)))<30)

[1] 257

> sum((as.integer(diff(created)))<10)

[1] 145

You could also think of these as ratios, for example 145/500 =

0.29. And where we have a ratio, we often can think about it

as a probability: There is a 29% probability that the next tweet

will arrive in 10 seconds or less. You could make a function to

create a whole list of these probabilities. Some sample code

for such a function appears at the end of the chapter. Some

new scripting skills that we have not yet covered (for exam-

ple, the "for loop") appear in this function, but try making

sense out of it to stretch your brain. Output from this function

created the plot that appears below.

This is a classic Poisson distribution of arrival probabilities.

The x-axis contains 10 second intervals (so by the time you

see the number 5 on the x-axis, we are already up to 50 sec-

onds). This is called a cumulative probability plot and you

read it by talking about the probability that the next tweet

will arrive in the amount of time indicated on the x-axis or

less. For example, the number ﬁve on the x-axis corresponds

to about a 60% probability on the y-axis, so there is a 60%

probability that the next tweet will arrive in 50 seconds or

less. Remember that this estimate applies only to the data in

this sample!

91

In the next chapter we will reexamine sampling in the context

of Poisson and learn how to compare two Poisson distribu-

tions to ﬁnd out which hashtag is more popular.

Let’s recap what we learned from this chapter. First, we have

begun to use the project features of R-studio to establish a

clean environment for each R project that we build. Second,

we used the source code window of R-studio to build two or

three very useful functions, ones that we will reuse in future

chapters. Third, we practiced the skill of installing packages

to extend the capabilities of R. Speciﬁcally, we loaded Jeff

Gentry’s twitteR package and the other three packages it de-

pends upon. Fourth, we put the twitteR package to work to

obtain our own fresh data right from the web. Fifth, we

started to condition that data, for example by creating a

sorted list of tweet arrival times. And ﬁnally, we started to

analyze and visualize those data, by conjecturing that this

sample of arrival times ﬁtted the classic Poisson distribution.

Chapter Challenge

Modify the TweetFrame() function created at the beginning of

this chapter to sort the dataframe based on the creation time

of the tweets. This will require taking the line of code from a

few pages ago that has the order() function in it and adding

this to the TweetFrame() function with a few minor modiﬁca-

tions. Here’s a hint: Create a temporary dataframe inside the

function and don’t attach it while you’re working with it.

You’ll need to use the $ notation to access the variable you

want to use to order the rows.

Sources

http://cran.r-project.org/web/packages/twitteR/twitteR.pdf

http://cran.r-project.org/web/packages/twitteR/vignettes/twitte

R.pdf

http://en.wikipedia.org/wiki/Ladislaus_Bortkiewicz

http://en.wikipedia.org/wiki/Poisson_distribution

http://hashtags.org/

http://www.inside-r.org/packages/cran/twitteR/docs/example

Oauth

http://www.khanacademy.org/math/probability/v/poisson-proc

ess-1

http://www.khanacademy.org/math/probability/v/poisson-proc

ess-2

https://support.twitter.com/articles/49309 (hashtags explained)

http://www.rdatamining.com/examples/text-mining

92

R Script - Create Vector of Probabilities From Arrival Times

# ArrivalProbability - Given a list of arrival times

# calculates the delays between them using lagged differences

# then computes a list of cumulative probabilities of arrival

# for the sequential list of time increments

# times - A sorted, ascending list of arrival times in POSIXct

# increment - the time increment for each new slot, e.g. 10 sec

# max - the highest time increment, e.g., 240 sec

#

# Returns - an ordered list of probabilities in a numeric vector

# suitable for plotting with plot()

ArrivalProbability<-function(times, increment, max)

{

# Initialize an empty vector

plist <- NULL

# Probability is defined over the size of this sample

# of arrival times

timeLen <- length(times)

# May not be necessary, but checks for input mistake

if (increment>max) {return(NULL)}

for (i in seq(increment, max, by=increment))

{

# diff() requires a sorted list of times

# diff() calculates the delays between neighboring times

# the logical test <i provides a list of TRUEs and FALSEs

# of length = timeLen, then sum() counts the TRUEs.

# Divide by timeLen to calculate a proportion

plist<-c(plist,(sum(as.integer(diff(times))<i))/timeLen)

}

return(plist)

}

R Functions Used in This Chapter

attach() - Makes the variables of a dataset available without $

as.integer() - Coerces data into integers

detach() - Undoes an attach function

diff() - Calculates differences between neighboring rows

do.call() - Calls a function with a variable number of arguments

function() - Deﬁnes a function for later use

hist() - Plots a histogram from a list of data

install.packages() - Downloads and prepares a package for use

lapply() - Applies a function to a list

library() - Loads a package for use; like require()

mean() - Calculates the arithmetic mean of a vector

median() - Finds the statistical center point of a list of numbers

93

mfv() - Most frequent value; part of the modeest() package

mode() - Shows the basic data type of an object

order() - Returns a sorted list of index numbers

rbind() - Binds rows into a dataframe object

require() - Tests if a package is loaded and loads it if needed

searchTwitter() - Part of the twitteR package

str() - Describes the structure of a data object

sum() - Adds up a list of numbers

94

In the previous chapter we found that arrival times of tweets on a given topic seem to ﬁt a Poisson

distribution. Armed with that knowledge we can now develop a test to compare two different Twitter

topics to see which one is more popular (or at least which one has a higher posting rate). We will use

our knowledge of sampling distributions to understand the logic of the test.

CHAPTER 11

95

Popularity Contest

Which topic on Twitter is more popular, Lady Gaga or Oprah Win-

frey? This may not seem like an important question, depending

upon your view of popular culture, but if we can make the com-

parison for these two topics, we can make it for any two topics. Cer-

tainly in the case of presidential elections, or a corruption scandal

in the local news, or an international crisis, it could be a worth-

while goal to be able to analyze social media in a systematic way.

And on the surface, the answer to the question seems trivial: Just

add up who has more tweets. Surprisingly, in order to answer the

question in an accurate and reliable way, this won’t work, at least

not very well. Instead, one must consider many of the vexing ques-

tions that made inferential statistics necessary.

Let’s say we retrieved one hour’s worth of Lady Gaga tweets and a

similar amount of Oprah Winfrey tweets and just counted them

up. What if it just happened to be a slow news day for Oprah? It

really wouldn’t be a fair comparison. What if most of Lady Gaga’s

tweets happen at midnight or on Saturdays? We could expand our

sampling time, maybe to a day or a week. This could certainly

help: Generally speaking, the bigger the sample, the more represen-

tative it is of the whole population, assuming it is not collected in a

biased way. This approach deﬁnes popularity as the number of

tweets over a ﬁxed period of time. Its success depends upon the

choice of a sufﬁciently large period of time, that the tweets are col-

lected for the two topics at the same time, and that the span of time

chosen happens to be equally favorable for both two topics.

Another approach to the popularity comparison would build upon

what we learned in the previous chapter about how arrival times

(and the delays between them) ﬁt into the Poisson distribution. In

this alternative deﬁnition of the popularity of a topic, we could sug-

gest that if the arrival curve is "steeper" for the ﬁrst topic in con-

trast to the second topic, then the ﬁrst topic is more active and

therefore more popular. Another way of saying the same thing is

that for the more popular topic, the likely delay until the arrival of

the next tweet is shorter than for the less popular topic. You could

also say that for a given interval of time, say ten minutes, the num-

ber of arrivals for the ﬁrst topic would be higher than for the sec-

ond topic. Assuming that the arrival delays ﬁt a Poisson distribu-

tion, these are all equivalent ways of capturing the comparison be-

tween the two topics.

Just as we did in the chapter entitled, "Sample in a Jar," we can use

a random number generator in R to illustrate these kinds of differ-

ences more concretely. The relevant function for the Poisson distri-

bution is rpois(), "random poisson." The rpois() function will gener-

ate a stream of random numbers that roughly ﬁt the Poisson distri-

bution. The ﬁt gets better as you ask for a larger and larger sample.

The ﬁrst argument to rpois() is how many random numbers you

want to generate and the second number is the average delay be-

tween arrivals that you want the random number generator to try

to come close to. We can look at a few of these numbers and then

use a histogram function to visualize the results:

> rpois(10,3)

[1] 5 4 4 2 0 3 6 2 3 3

> mean(rpois(100,3))

[1] 2.99

> var(rpois(100,3))

[1] 3.028182

96

> hist(rpois(1000,3))

In the ﬁrst command above, we generate a small sample of n=10

arrival delays, with a hoped for mean of 3 seconds of delay, just to

see what kind of numbers we get. You can see that all of the num-

bers are small integers, ranging from 0 to 6. In the second com-

mand we double check these results with a slightly larger sample

of n=100 to see if rpois() will hit the mean we asked for. In that run

it came out to 2.99, which was pretty darned close. If you run this

command yourself you will ﬁnd that your result will vary a bit

each time: it will sometimes be slightly larger than three and occa-

sionally a little less than three (or whatever mean you specify).

This is normal, because of the random number generator. In the

third command we run yet another sample of 100 random data

points, this time analyzing them with the var() function (which cal-

culates the variance; see the chapter entitled "Beer, Farms, and

Peas"). It is a curious fact of Poission distributions that the mean

and the variance of the "ideal" (i.e., the theoretical) distribution are

the same. In practice, for a small sample, they may be different.

In the ﬁnal command, we ask for a histogram of an even larger

sample of n=1000. The histogram shows the most common value

hanging right around three seconds of delay with a nice tail that

points rightwards and out to about 10 seconds of delay. You can

think of this as one possible example of what you might observe of

the average delay time between tweets was about three seconds.

Note how similar the shape of this histogram is to what we ob-

served with real tweets in the last chapter.

Compare the histogram on the previous page to the one on the

next page that was generated with this command:

hist(rpois(1000,10))

It is pretty easy to see the different shape and position of this histo-

gram, which has a mean arrival delay of about ten seconds. First of

all, there are not nearly as many zero length delays. Secondly, the

most frequent value is now about 10 (as opposed to two in the pre-

vious histogram). Finally, the longest delay is now over 20 seconds

(instead of 10 for the previous histogram). One other thing to try is

this:

> sum(rpois(1000,10)<=10)

[1] 597

97

This command generated 1000 new random numbers, following

the Poisson distribution and also with a hoped-for mean of 10, just

like in the histogram on the next page. Using the "<=" inequality

test and the sum() function, we then counted up how many events

were less than or equal to 12, and this turned out to be 597 events.

As a fraction of the total of n=1000 data points that rpois() gener-

ated, that is 0.597, or 59.7%.

98

Review 11.1 Popularity Contest (Mid-Chapter Review)

Check Answer

Question 1 of 4

The Poisson distribution has a characteristic shape that

would be described as:

A.

Negatively (left) skewed

B.

Positively (right) skewed

C.

Symmetric (not skewed)

D. None of the above

We can look at the same kind of data in terms of the probability of

arrival within a certain amount of time. Because rpois() generates

delay times directly (rather than us having to calculate them from

neighboring arrival times), we will need a slightly different func-

tion than the ArrivalProbabilities() that we wrote and used in the

previous chapter. We’ll call this function "DelayProbability" (the

code is at the end of this chapter):

> DelayProbability(rpois(100,10),1,20)

[1] 0.00 0.00 0.00 0.03 0.06 0.09 0.21 0.33 0.48

0.61 0.73 0.82 0.92

[14] 0.96 0.97 0.98 0.99 1.00 1.00 1.00

At the heart of that command is the rpois() function, requesting

100 points with a mean of 10. The other two parameters are the in-

crement, in this case one second, and the maximum delay time, in

this case 20 seconds. The output from this function is a sorted list

of cumulative probabilities for the times ranging from 1 second to

20 seconds. Of course, what we would really like to do is compare

these probabilities to those we would get if the average delay was

three seconds instead of ten seconds. We’re going to use two cool

tricks for creating this next plot. First, we will use the points() com-

mand to add points to an existing plot. Second, we will use the

col= parameter to specify two different colors for the points that

we plot. Here’s the code that creates a plot and then adds more

points to it:

> plot(DelayProbability(rpois(100,10),1,20), col=2)

> points(DelayProbability(rpois(100,3),1,20), col=3)

Again, the heart of each of these lines of code is the rpois() function

that is generating random Poisson arrival delays for us. Our pa-

rameters for increment (1 second) and maximum (20 seconds) are

the same for both lines. The ﬁrst line uses col=2, which gives us red

points, and the second gives us col=3, which yields green points:

This plot clearly shows that the green points have a "steeper" pro-

ﬁle. We are more likely to have earlier arrivals for the 3-second de-

lay data than we are for the 10-second data. If these were real

tweets, the green tweets would be piling in much faster than the

red tweets. Here’s a reminder on how to read this plot: Look at a

value on the X-axis, for example "5." Then look where the dot is

99

and trace leftward to the Y-axis. For the red dot, the probability

value at time (x) equal 4 is about 0.10. So for the red data there is

about a 10% chance that the next event will occur within ﬁve time

units (we’ve been calling them seconds, but they could really be

anything, as long as you use the units consistently throughout the

whole example). For the green data there is about a 85% chance

that the next event will occur within four time units. The fact that

the green curve rises more steeply than the red curve means that

for these two samples only the green stuff is arriving much more often

than the red stuff.

These reason we emphasized the point "for these samples only" is

that we know from prior chapters that every sample of data you

collect varies by at least a little bit and sometimes by quite a lot. A

sample is just a snapshot, after all, and things can and do change

from sample to sample. We can illustrate this by running and plot-

ting multiple samples, much as we did in the earlier chapter:

> plot(DelayProbability(rpois(100,10),1,20))

> for (i in 1:15) {points(DelayProbability(r-

pois(100,10),1,20))}

This is the ﬁrst time we have used the "for loop" in R, so let’s walk

through it. A "for loop" is one of the basic constructions that com-

puter scientists use to "iterate" or repeatedly run a chunk of code.

In R, a for loop runs the code that is between the curly braces a cer-

tain number of times. The number of times R runs the code de-

pends on the expression inside the parentheses that immediately

follow the "for."

In the example above, the expression "i in 1:15" creates a new data

object, called i, and then puts the number 1 in it. Then, the for loop

keeps adding one to the value of i, until i reaches 15. Each time that

it does this, it runs the code between the curly braces. The expres-

sion "in 1:15" tells R to start with one and count up to 15. The data

object i, which is just a plain old integer, could also have been used

within the curly braces if we had needed it, but it doesn’t have to

be used within the curly braces if it is not needed. In this case we

didn’t need it. The code inside the curly braces just runs a new ran-

dom sample of 100 Poisson points with a hoped for mean of 10.

When you consider the two command lines on the previous page

together you can see that we initiate a plot() on the ﬁrst line of

100

code, using similar parameters to before (random poisson numbers

with a mean of 10, fed into our probability calculator, which goes

in increments of 1 second up to 20 seconds). In the second line we

add more points to the same plot, by running exactly 15 additional

copies of the same code. Using rpois() ensures that we have new

random numbers each time:

Now instead of just one smooth curve we have a bunch of curves,

and that these curves vary quite a lot. In fact, if we take the exam-

ple of 10 seconds (on the X-axis), we can see that in one case the

probability of a new event in 10 seconds could be as low as 0.50,

while in another case the probability is as high as about 0.70.

This shows why we can’t just rely on one sample for making our

judgments. We need to know something about the uncertainty that

surrounds a given sample. Fortunately, R gives us additional tools

to help us ﬁgure this situation out. First of all, even though we had

loads of fun programming the DelayProbability() function, there is

a quicker way to get information about what we ideally expect

from a Poisson distribution. The function ppois() gives us the theo-

retical probability of observing a certain delay time, given a particu-

lar mean. For example:

> ppois(3, lambda=10)

[1] 0.01033605

So you can read this as: There is a 1% chance of observing a delay

of 3 or less in a Poisson distribution with mean equal to 10. Note

that in statistical terminology, "lambda" is the term used for the

mean of a Poisson distribution. We’ve provided the named parame-

ter "lambda=10" in the example above just to make sure that R

does not get confused about what parameter we are controlling

when we say "10." The ppois() function does have other parame-

ters that we have not used here. Now, using a for loop, we could

get a list of several of these theoretical probabilities:

> plot(1,20,xlim=c(0,20),ylim=c(0,1))

> for (i in 1:20) {points(i,ppois(i,lambda=10)) }

We are using a little code trick in the ﬁrst command line above by

creating a nearly empty set of axes with the plot() function, and

then ﬁlling in the points in the second line using the points() func-

tion. This gives the following plot:

You may notice that this plot looks a lot like the ones earlier in this

101

chapter as well as somewhat similar to the probability plot in the

previous chapter. When we say the "theoretical distribution" we

are talking about the ideal Poisson distribution that would be gen-

erated by the complex equation that Mr. Poisson invented a couple

of centuries ago. Another way to think about it is this: Instead of

just having a small sample of points, which we know has a lot of

randomness in it, what if we had a truly humongous sample with

zillions of data points? The curve in the plot above is just about

what we would observe for a truly humongous sample (where

most of the biases up or down cancel themselves out because the

large number of points).

So this is the ideal, based on the mathematical theory of the Pois-

son distribution, or what we would be likely to observe if we cre-

ated a really large sample. We know that real samples, of reason-

able amounts of data, like 100 points or 1000 points or even 10,000

points, will not hit the ideal exactly, because some samples will

come out a little higher and others a little lower.

We also know, from the histograms and output earlier in the chap-

ter, that we can look at the mean of a sample, or the count of events

less than or equal to the mean, or the arrival probabilities in the

graph on this page, and in each case we are looking at different versions

of the same information. Check out these ﬁve commands:

> mean(rpois(100000,10))

[1] 10.01009

> var(rpois(100000,10))

[1] 10.02214

> sum(rpois(100000,10)<=10)/100000

[1] 0.58638

> ppois(10,lambda=10)

[1] 0.58303

> qpois(0.58303,lambda=10)

[1] 10

In the ﬁrst command, we conﬁrm that for a very large random sam-

ple of n=100,000 with a desired mean of 10, the actual mean of the

random sample is almost exactly 10. Likewise, for another large

random sample with a desired mean of 10, the variance is 10. In

the next command, we use the inequality test and the sum() func-

tion again to learn that the probability of observing a value of 10 or

less in a very large sample is about 0.59 (note that the sum() func-

tion yielded 58,638 and we divided by 100,000 to get the reported

value of 0.58638). Likewise, when we ask for the theoretical distri-

bution with ppois() of observing 10 or less in a sample with a mean

of 10, we get a probability of 0.58303, which is darned close to the

empirical result from the previous command. Finally, if we ask

qpois() what is the threshold value for a probability of 0.58303 is in a

Poisson sample with mean of 10, we get back the answer: 10. You

may see that qpois() does the reverse of what ppois() does. For fun,

try this formula on the R command line: !

! qpois(ppois(10, lambda=10), lambda=10)

Here’s one last point to cap off this thinking. Even with a sample of

100,000 there is some variation in samples. That’s why the 0.58638

from the sum() function above does not exactly match the theoreti-

cal 0.58303 from the ppois() function above. We can ask R to tell us

how much variation there is around one of these probabilities us-

ing the poisson.test() function like this:

102

> poisson.test(58638, 100000)

95 percent confidence interval:

0.5816434 0.5911456

We’ve truncated a little of the output in the interests of space: What

you have left is the upper and lower bounds on a 95% conﬁdence

interval. Here’s what a conﬁdence interval is: For 95% of the sam-

ples that we could generate using rpois(), using a sample size of

100,000, and a desired mean of 10, we will get a result that lies be-

tween 0.5816434 and 0.5911456 (remember that this resulting pro-

portion is calculated as the total number of events whose delay

time is 10 or less). So we know what would happen for 95% of the

rpois() samples, but the assumption that statisticians also make is

that if a natural phenomenon, like the arrival time of tweets, also

ﬁts the Poisson distribution, that this same conﬁdence interval

would be operative. So while we know that we got 0.58638 in one

sample on the previous page, it is likely that future samples will

vary by a little bit (about 1%). Just to get a feel for what happens to

the conﬁdence interval with smaller samples, look at these:

> poisson.test(5863, 10000)

95 percent confidence interval:

0.5713874 0.6015033

> poisson.test(586, 1000)

95 percent confidence interval:

0.5395084 0.6354261

> poisson.test(58, 100)

95 percent confidence interval:

0.4404183 0.7497845

We’ve bolded the parameters that changed in each of the three com-

mands above, just to emphasize that in each case we’ve reduced

the sample size by a factor of 10. By the time we get to the bottom

look how wide the conﬁdence interval gets. With a sample of 100

events, of which 58 had delays of 10 seconds or less, the conﬁdence

interval around the proportion of 0.58 ranges from a low of 0.44 to

a high of 0.75! That’s huge! The conﬁdence interval gets wider and

wider as we get less and less conﬁdent about the accuracy of our esti-

mate. In the case of a small sample of 100 events, the conﬁdence in-

terval is very wide, showing that we have a lot of uncertainty

about our estimate that 58 events out of 100 will have arrival de-

lays of 10 or less. Note that you can ﬁlter out the rest of the stuff

that poisson.test() generates by asking speciﬁcally for the "conf.int"

in the output that is returned:

> poisson.test(58, 100)$conf.int

[1] 0.4404183 0.7497845

attr(,"conf.level")

[1] 0.95

The bolded part of the command line above shows how we used

the $ notation to get a report of just the bit of output that we

wanted from poisson.test(). This output reports the exact same con-

ﬁdence interval that we saw on the previous page, along with a re-

minder in the ﬁnal two lines that we are looking at a 95% conﬁ-

dence interval.

103

At this point we have all of the knowledge and tools we need to

compare two sets of arrival rates. Let’s grab a couple of sets of

tweets and extract the information we need. First, we will use the

function we created in the last chapter to grab the ﬁrst set of

tweets:

tweetDF <- TweetFrame("#ladygaga",500)

Next, we need to sort the tweets by arrival time, That is, of course,

unless you accepted the Chapter Challenge in the previous chapter

and built the sorting into your TweetFrame() function.

sortweetDF<-tweetDF[order(as.integer( + "

tweetDF$created)), ]

Now, we’ll extract a vector of the time differences. In the previous

chapter the use of the diff() function occurred within the Arrival-

Probability() function that we developed. Here we will use it di-

rectly and save the result in a vector:

eventDelays<- + "

as.integer(diff(sortweetDF$created))

Now we can calculate a few of the things we need in order to get a

picture of the arrival delays for Lady Gaga’s tweets:

> mean(eventDelays)

[1] 30.53707

> sum(eventDelays<=31)

[1] 333

So, for Lady Gaga tweets, the mean arrival delay for the next tweet

is just short of 31 seconds. Another way of looking at that same sta-

tistic is that 333 out of 500 tweets (0.666, about two thirds) arrived

within 31 seconds of the previous tweet. We can also ask

poisson.test() to show us the conﬁdence interval around that value:

> poisson.test(333,500)$conf.int

[1] 0.5963808 0.7415144

attr(,"conf.level")

[1] 0.95

So, this result suggests that for 95% of the Lady Gaga samples of

tweets that we might pull from the Twitter system, the proportion

arriving in 31 seconds or less would fall in this conﬁdence band. In

other words, we’re not very likely to see a sample with a propor-

tion under 59.6% or over 74.1%. That’s a pretty wide band, so we

do not have a lot of exactitude here.

Now let’s get the same data for Oprah:

> tweetDF <- TweetFrame("#oprah",500)

> sortweetDF<-tweetDF[order( + "

as.integer(tweetDF$created)), ]

> eventDelays<- +"

as.integer(diff(sortweetDF$created))

> mean(eventDelays)

[1] 423.01

Hmm, I guess we know who is boss here! Now let’s ﬁnish the job:

> sum(eventDelays<=31)

[1] 73

104

> poisson.test(73,500)$conf.int

[1] 0.1144407 0.1835731

attr(,"conf.level")

[1] 0.95

The sum() function, above, calculates that only 73 out of Oprah’s

sample of 500 tweets arrive in an interval of 31 or less. We use 31,

the mean of the Lady Gaga sample, because we need to have a com-

mon basis of comparison. So for Oprah, the proportion of events

that occur in the 31 second timeframe is, 73/500 = 0.146, or about

14.6%. That’s a lot lower than the 66.6% of Lady Gaga tweets, for

sure, but we need to look at the conﬁdence interval around that

value. So the poisson.test() function just above for Oprah reports

that the 95% conﬁdence interval runs from about 11.4% to 18.4%.

Note that this conﬁdence interval does not overlap at all with the

conﬁdence interval for Lady Gaga, so we have a very strong sense

that these two rates are statistically quite distinctive - in other

words, this is a difference that was not caused by the random inﬂu-

ences that sampling always creates. We can make a bar graph to

summarize these differences. We’ll use the barplot2() function,

which is in a package called gplots(). If you created the EnsurePack-

age() function a couple of chapters ago, you can use that. Other-

wise make sure to load gplots manually:

> EnsurePackage("gplots")

> barplot2(c(0.666,0.146), + "

!!ci.l=c(0.596,0.114), + "

!!ci.u=c(0.742,0.184), +"

!!plot.ci=TRUE, +"

!!names.arg=c("Gaga","Oprah"))

This is not a particularly efﬁcient way to use the barplots() func-

tion, because we are supplying our data by typing it in, using the

c() function to create short vectors of values on the command line.

On the ﬁrst line,, we supply a list of the means from the two sam-

ples, expressed as proportions. On the next two lines we ﬁrst pro-

vide the lower limits of the conﬁdence intervals and then the up-

per limits. The plot.ci=TRUE parameter asks barplot2() to put conﬁ-

dence interval whiskers on each bar. The ﬁnal line provides labels

to put underneath the bars. Here’s what we get:

105

This is not an especially attractive bar plot, but it does represent

the information we wanted to display accurately. And with the as-

sistance of this plot, it is easy to see both the substantial difference

between the two bars and the fact that the conﬁdence intervals do

not overlap.

For one ﬁnal conﬁrmation of our results, we can ask the

poisson.text() function to evaluate our two samples together. This

code provides the same information to poisson.test() as before, but

now provides the event counts as short lists describing the two

samples, with 333 events (under 31 seconds) for Lady Gaga and 73

events for Oprah, in both cases out of 500 events:

> poisson.test(c(333,73),c(500,500))

! Comparison of Poisson rates

data: c(333, 73) time base: c(500, 500)

count1 = 333, expected count1 = 203, p-value <

2.2e-16

alternative hypothesis: true rate ratio is not

equal to 1

95 percent confidence interval:

3.531401 5.960511

sample estimates:

rate ratio

4.561644

Let’s walk through this output line by line. Right after the com-

mand, we get a brief conﬁrmation from the function that we’re

comparing two event rates in this test rather than just evaluating a

single rate: "Comparison of Poisson rates." The next line conﬁrms

the data we provided. The next line, that begins with "count1 =

333" conﬁrms the basis of of the comparison and then shows a

"pooled" count that is the weighted average of 333 and 73. The p-

value on that same line represents the position of a probability tail

for "false positives." Together with the information on the next line,

"alternative hypothesis," this constitutes what statisticians call a

"null hypothesis signiﬁcance test." Although this is widely used in

academic research, it contains less useful information than conﬁ-

dence intervals and we will ignore it for now.

106

The next line, "95% conﬁdence interval," is a label for the most im-

portant information, which is on the line that follows. The values

of 3.53 and 5.96 represent the upper and lower limits of the 95%

conﬁdence interval around the observed rate ratio of 4.56 (reported on

the ﬁnal line). So, for 95% of samples that we might draw from twit-

ter, the ratio of the Gaga/Oprah rates might be as low as 3.53 and

as high as 5.96. So we can be pretty sure (95% conﬁdence) that

Lady Gaga gets tweets at least 3.5 times as fast as Oprah. Because

the conﬁdence interval does not include 1, which would be the

same thing as saying that the two rates are identical, we can be

pretty certain that the observed rate ratio of 4.56 is not a statistical

ﬂuke.

For this comparison, we chose two topics that had very distinctive

event rates. As the bar chart on the previous page attests, there was

a substantial difference between the two samples in the rates of arri-

val of new tweets. The statistical test conﬁrmed this for us, and al-

though the ability to calculate and visualize the conﬁdence inter-

vals was helpful, we probably could have guessed that such a large

difference over a total of 1000 tweets was not a result due to sam-

pling error.

With other topics and other comparisons, however, the results will

not be as clear cut. After completing the chapter challenge on the

next page, we compared the "#obama" hashtag to the "#romney"

hashtag. Over samples of 250 tweets each, Obama had 159 events

at or under the mean, while Romney had only 128, for a ratio of

1.24 in Obama’s favor. The conﬁdence interval told a different

story, however: the lower bound of the conﬁdence interval was

0.978, very close to, but slightly below one. This signiﬁes that we

can’t rule out the possibility that the two rates are, in fact, equal

and that the slightly higher rate (1.24 to 1) that we observed for

Obama in this one sample might have come about due to sampling

error. When a conﬁdence interval overlaps the point where we con-

sider something to be a "null result" (in this case a ratio of 1:1) we

have to take seriously the possibility that peculiarities of the sam-

ple(s) we drew created the observed difference, and that a new set

of samples might show the opposite of what we found this time.

Chapter Challenge

Write a function that takes two search strings as arguments and

that returns the results of a Poisson rate ratio test on the arrival

rates of tweets on the two topics. Your function should ﬁrst run the

necessary Twitter searches, then sort the tweets by ascending time

of arrival and calculate the two vectors of time differentials. Use

the mean of one of these vectors as the basis for comparison and

for each vector, count how many events are at or below the mean.

Use this information and the numbers of tweets requested to run

the poisson.test() rate comparison.

Sources

Barplots

http://addictedtor.free.fr/graphiques/RGraphGallery.php?graph

=54

http://biostat.mc.vanderbilt.edu/twiki/pub/Main/StatGraphCo

urse/graphscourse.pdf

http://rgm2.lab.nig.ac.jp/RGM2/func.php?rd_id=gplots:barplot2

107

Poisson Distribution

http://books.google.com/books?id=ZKswvkqhygYC&printsec=fr

ontcover

http://www.khanacademy.org/math/probability/v/poisson-proc

ess-1

http://www.khanacademy.org/math/probability/v/poisson-proc

ess-2

http://stat.ethz.ch/R-manual/R-patched/library/stats/html/Poi

sson.html

http://stat.ethz.ch/R-manual/R-patched/library/stats/html/poi

sson.test.html

http://stats.stackexchange.com/questions/10926/how-to-calculat

e-conﬁdence-interval-for-count-data-in-r

http://www.computing.dcu.ie/~mbezbradica/teaching/CA266/

CA266_13_Poisson_Distribution.pdf

R Functions Used in this Chapter

as.integer() - Coerces another data type to integer if possible

barplot2() - Creates a bar graph

c() - Concatenates items to make a list

diff() - Calculates time difference on neighboring cases

EnsurePackage() - Custom function, install() and require() package

for() - Creates a loop, repeating execution of code

hist() - Creates a frequency histogram

mean() - Calculates the arithmetic mean

order() - Provides a list of indices reﬂecting a new sort order

plot() - Begins an X-Y plot

points() - Adds points to a plot started with plot()

poisson.test() - Conﬁdence intervals for poisson events or ratios

ppois() - Returns a cumulative probability for particular threshold

qpois() - Does the inverse of ppois(): Probability into threshold

rpois() - Generates random numbers ﬁtting a Poisson distribution

sum() - Adds together a list of numbers

TweetFrame() - Custom procedure yielding a dataset of tweets

var() - Calculates variance of a list of numbers

108

R Script - Create Vector of Probabilities From Delay Times

# Like ArrivalProbability, but works with unsorted list

# of delay times

DelayProbability<-function(delays, increment, max)

{

# Initialize an empty vector

plist <- NULL

# Probability is defined over the size of this sample

# of arrival times

delayLen <- length(delays)

# May not be necessary, but checks for input mistake

if (increment>max) {return(NULL)}

for (i in seq(increment, max, by=increment))

{

# logical test <=i provides list of TRUEs and FALSEs

# of length = timeLen, then sum() counts the TRUEs

plist<-c(plist,(sum(delays<=i)/delayLen))

}

return(plist)

}

109

Prior chapters focused on statistical analysis of tweet arrival times and built on earlier knowledge of

samples and distributions. This chapter switches gears to focus on manipulating so-called

"unstructured" data, which in most cases means natural language texts. Tweets are again a useful

source of data for this because tweets are mainly a short (140 characters or less) character strings.

CHAPTER 12

110

String Theory

Yoiks, that last chapter was very challenging! Lots of numbers, lots

of statistical concepts, lots of graphs. Let’s take a break from all

that (temporarily) and focus on a different kind of data for a while.

If you think about the Internet, and speciﬁcally about the World

Wide Web for a while, you will realize: 1) That there are zillions of

web pages; and 2) That most of the information on those web

pages is "unstructured," in the sense that it does not consist of nice

rows and columns of numeric data with measurements of time or

other attributes. Instead, most of the data spread out across the

Internet is text, digital photographs, or digital videos. These last

two categories are interesting, but we will have to postpone consid-

eration of them while we consider the question of text.

Text is, of course, one of the most common forms of human commu-

nication, hence the label that researchers use sometimes: natural

language. When we say natural language text we mean words cre-

ated by humans and for humans. With our cool computer technol-

ogy, we have collectively built lots of ways of dealing with natural

language text. At the most basic level, we have a great system for

representing individual characters of text inside of computers

called "Unicode." Among other things Unicode provides for a bi-

nary representation of characters in most of the world’s written lan-

guages, over 110,000 characters in all. Unicode supersedes ASCII

(the American Standard Code for Information Interchange), which

was one of the most popular standards (especially in the U.S.) for

representing characters from the dawn of the computer age.

With the help of Unicode, most computer operating systems, and

most application programs that handle text have a core strategy for

representing text as lists of binary codes. Such lists are commonly

referred to as "character strings" or in most cases just "strings." One

of the most striking things about strings from a computer program-

ming perspective is that they seem to be changing their length all

the time. You can’t perform the usual mathematical operations on

character strings the way you can with numbers - no multiplica-

tion or division - but it is very common to "split" strings into

smaller strings, and to "add" strings together to form longer

strings. So while we may start out with, "the quick brown fox," we

may end up with "the quick brown" in one string and "fox" in an-

other, or we may end up with something longer like, "the quick

brown fox jumped over the lazy dog."

Fortunately, R, like most other data handling applications, has a

wide range of functions for manipulating, keeping track of, search-

ing, and even analyzing string data. In this chapter, we will use our

budding skills working with tweet data to learn the essentials of

working with unstructured text data. The learning goal here is sim-

ply to become comfortable with examining and manipulating text

data. We need these basic skills before we can tackle a more inter-

esting problem.

Let’s begin by loading a new package, called "stringr". Although R

has quite a few string functions in its core, they tend to be a bit dis-

organized. So Hadley Wickham, a professor of statistics at Rice Uni-

versity, created this "stringr" package to make a set of string ma-

nipulation functions a bit easier to use and more comprehensive.

You can install() and library() this package using the point and

click features of R-Studio (look in the lower right hand pane under

the Packages tab), or if you created the EnsurePackage() function

from a couple of chapters back, you can use that:

EnsurePackage("stringr")

111

Now we can grab a new set of tweets with our custom function

TweetFrame() from a couple of chapters ago (if you need the code,

look in the chapter entitled "Tweet, Tweet"; we’ve also pasted the

enhanced function, that sorts the tweets into arrival order, into the

end of this chapter):

tweetDF <- TweetFrame("#solar",100)

This command should return a data frame containing about 100

tweets, mainly having to do with solar energy. You can choose any

topic you like - all of the string techniques we examine in this chap-

ter are widely applicable to any text strings. We should get ori-

ented by taking a look at what we retrieved. The head() function

can return the ﬁrst entries in any vector or list:

We provide a screen shot from R-Studio here just to preserve the

formatting of this output. In the left hand margin, the number 97

represents R’s indexing of the original order in which the tweet

was received. The tweets were re-sorted into arrival order by our

enhanced TweetFrame() function (see the end of the chapter for

code). So this is the ﬁrst element in our dataframe, but internally R

has numbered it as 97 out of the 100 tweets we obtained. On the

ﬁrst line of the output, R has place the label "text" and this is the

ﬁeld name of the column in the dataframe that contains the texts of

the tweets. Other dataframe ﬁelds that we will not be using in this

chapter include: "favorited," "replyToSN," and "truncated." You

may also recognize the ﬁeld name "created" which contains the PO-

SIX format time and date stamp that we used in previous chapters.

Generally speaking, R has placed the example data

(from tweet 97) that goes with the ﬁeld name just under-

neath it, but the text justiﬁcation can be confusing, and

it makes this display very hard to read. For example,

there is a really long number that starts with "1908" that

is the unique numeric identiﬁer (a kind of serial num-

ber) for this tweet. The ﬁeld name "id" appears just

above it, but is right justiﬁed (probably because the

ﬁeld is a number). The most important fact for us to

note is that if we want to work with the text string that

is the tweet itself, we need to use the ﬁeld name "text."

Let’s see if we can get a somewhat better view if we use

the head() function just on the text ﬁeld. This command

should provide just the ﬁrst 2 entries in the "text" col-

umn of the dataframe:

head(tweetDF$text,2)

[1] "If your energy needs increase after you in-

stall a #solar system can you upgrade? Our ex-

perts have the answer! http://t.co/ims8gDWW"

112

[2] "#green New solar farms in West Tennessee

signal growth: Two new solar energy farms produc-

ing electricity ... http://t.co/37PKAF3N #solar"

A couple of things which will probably seem obvious, but are none-

theless important to point out: The [1] and [2] are not part of the

tweet, but are the typical line numbers that R uses in its output.

The actual tweet text is between the double quotes. You can see the

hashtag "#solar" appears in both tweets, which makes sense be-

cause this was our search term. There is also a second hashtag in

the ﬁrst tweet "#green" so we will have to be on the lookout for

multiple hashtags. There is also a "shortened" URL in each of these

tweets. If a Twitter user pastes in the URL of a website to which

they want to refer people, the Twitter software automatically short-

ens the URL to something that begins with "http://t.co/" in order

to save space in the tweet.

An even better way to look at these data, including the text and the

other ﬁelds is to use the data browser that is built into R-Studio. If

you look in the upper right hand pane of R-Studio, and make sure

that the Workspace tab is clicked, you should see a list of available

dataframes, under the heading "Data." One of these should be

"tweetDF." If you click on tweetDF, the data browser will open in

the upper left hand pane of R-Studio and you should be able to see

the ﬁrst ﬁeld or two of the ﬁrst dozen rows. Here’s a screen shot:

This screen shot conﬁrms what we observed in the command line

output, but gives us a much more appealing and convenient way

of looking through our data. Before we start to manipulate our

strings, let’s attach() tweetDF so that we don’t have to keep using

the $ notation to access the text ﬁeld. And before that, let’s check

what is already attached with the search() function:

> search()

[1] ".GlobalEnv" "sortweetDF" "package:gplots"

[4] "package:KernSmooth" "package:grid" "package:caTools"

We’ve truncated this list to save space, but you can see on the ﬁrst

line "sortweetDF" left over from our work in a previous chapter.

The other entries are all function packages that we want to keep ac-

tive. So let’s detach() sortweetDF and attach tweetDF:

> detach(sortweetDF)

> attach(tweetDF)

These commands should yield no additional output. If

you get any messages about "The following object(s)

are masked from..." you should run search() again and

look for other dataframes that should be detached be-

fore proceeding. Once you can run attach("tweetDF")

without any warnings, you can be sure that the ﬁelds

in this dataframe are ready to use without interference.

113

The ﬁrst and most basic thing to do with strings is to see how long

they are. The stringr package gives us the str_length() function to

accomplish this task:

> str_length(text)

[1] 130 136 136 128 98 75 131 139 85 157 107 49 75 139 136 136

[17] 136 72 73 136 157 123 160 142 142 122 122 122 122 134 82 87

[33] 89 118 94 74 103 91 136 136 151 136 139 135 70 122 122 136

[49] 123 111 83 136 137 85 154 114 117 98 125 138 107 92 140 119

[65] 92 125 84 81 107 107 73 73 138 63 137 139 131 136 120 124

[81] 124 114 78 118 138 138 116 112 101 94 153 79 79 125 125 102

[97] 102 139 138 153

These are the string lengths of the texts as reported to the com-

mand line. It is interesting to ﬁnd that there are a few of them (like

the very last one) that are longer than 140 characters:

> tail(text,1)

[1] "RT @SolarFred: Hey, #solar & wind people.

Tell @SpeakerBoehner and @Reuters that YOU have a

green job and proud to be providing energy Inde-

pendence to US"

As you can see, the tail() command works like the head() com-

mand except from the bottom up rather than the top down. So we

have learned that under certain circumstances Twitter apparently

does allow tweets longer than 140 characters. Perhaps the initial

phrase "RT @SolarFred" does not count against the total. By the

way "RT" stands for "retweet" and it indicates when the receiver of

a tweet has passed along the same message to his or her followers.

We can glue the string lengths onto the respective rows in the data-

frame by creating a new ﬁeld/column:

tweetDF$textlen <- str_length(text)

After running this line of text, you should use the data browser in

R-studio to conﬁrm that the tweetDF now has a new column of

data labeled "textlen". You will ﬁnd the new column all the way on

the rightmost side of the dataframe structure. One peculiarity of

the way R treats attached data is that you will not be able to access

the new ﬁeld without the $ notation unless you detach() and then

again attach() the data frame. One advantage of grafting this new

ﬁeld onto our existing dataframe is that we can use it to probe the

dataframe structure:

> detach(tweetDF)

> attach(tweetDF)

> tweetDF[textlen>140, "text"]

[1] "RT @andyschonberger: Exciting (and tempting)

to see #EVs all over the #GLS12 show. Combine EVs

w #solar generation and we have a winner!

http://t.co/NVsfq4G3"

We’ve truncated the output to save space, but in the data we are us-

ing here, there were nine tweets with lengths greater than 140. Not

all of them had "RT" in them, though, so the mystery remains. An

important word about the ﬁnal command line above, though:

We’re using the square brackets notation to access the elements of

tweetDF. In the ﬁrst entry, "textlen>140", we’re using a conditional

expression to control which rows are reported. Only those rows

where our new ﬁeld "textlen" contains a quantity larger than 140

114

will be reported to the output. In the second entry within square

brackets, "text" controls which columns are reported onto the out-

put. The square bracket notation is extremely powerful and some-

times a little unpredictable and confusing, so it is worth experi-

menting with. For example, how would you change that last com-

mand above to report all of the columns/ﬁelds for the matching

rows? Or how would you request the "screenName" column in-

stead of the "text" column? What would happen if you substituted

the number 1 in place of "text" on that command?

The next common task in working with strings is to count the num-

ber of words as well as the number of other interesting elements

within the text. Counting the words can be accomplished in several

ways. One of the simplest ways is to count the separators between

the words - these are generally spaces. We need to be careful not to

over count, if someone has mistakenly typed two spaces between a

word, so let’s make sure to take out doubles. The str_replace_all()

function from stringr can be used to accomplish this:

> tweetDF$modtext <- str_replace_all(text," "," ")

> tweetDF$textlen2 <- str_length(tweetDF$modtext)

> detach(tweetDF)

> attach(tweetDF)

> tweetDF[textlen != textlen2,]

The ﬁrst line above uses the str_replace_all() function to substitute

the one string in place of another as many times as the matching

string appears in the input. Three arguments appear on the func-

tion above: the ﬁrst is the input string, and that is tweetDF$text (al-

though we’ve referred to it just as "text because the dataframe is at-

tached). The second argument is the string to look for and the third

argument is the string to substitute in place of the ﬁrst. Note that

here we are asking to substitute one space any time that two in a

row are found. Almost all computer languages have a function

similar to this, although many of them only supply a function that

replaces the ﬁrst instance of the matching string.

In the second command we have calculated a new string length

variable based on the length of the strings where the substitutions

have occurred. We preserved this in a new variable/ﬁeld/column

so that we can compare it to the original string length in the ﬁnal

command. Note the use of the bracket notation in R to address a

certain subset of rows based on where the inequality is true. So

here we are looking for a report back of all of the strings whose

lengths changed. In the tweet data we are using here, the output

indicated that there were seven strings that had their length re-

duced by the elimination of duplicate spaces.

Now we are ready to count the number of words in each tweet us-

ing the str_count() function. If you give it some thought, it should

be clear that generally there is one more word than there are

spaces. For instance, in the sentence, "Go for it," there are two

spaces but three words. So if we want to have an accurate count,

we should add one to the total that we obtain from the str_count()

function:

> tweetDF$wordCount<-(str_count(modtext," ") + 1)

> detach(tweetDF)

> attach(tweetDF)

> mean(wordCount)

115

[1] 14.24

In this last command, we’ve asked R to report the mean value of

the vector of word counts, and we learn that on average a tweet in

our dataset has about 14 words in it.

Next, let’s do a bit of what computer scientists (and others) call

"parsing." Parsing is the process of dividing a larger unit, like a sen-

tence, into smaller units, like words, based on some kind of rule. In

many cases, parsing requires careful use of pattern matching. Most

computer languages accomplish pattern matching through the use

of a strategy called "regular expressions." A regular expression is a

set of symbols used to match patterns. For example, [a-z] is used to

match any lowercase letter and the asterisk is used to represent a

sequence of zero or more characters. So the regular expression "[a-

z]*" means, "match a sequence of zero or more lowercase charac-

ters.

If we wanted to parse the retweet sequence that appears at the be-

ginning of some of the tweets, we might use a regular expression

like this: "RT @[a-z,A-Z]*: ". Each character up to the square

bracket is a "literal" that has to match exactly. Then the "[a-z,A-Z]*"

lets us match any sequence of uppercase and lowercase characters.

Finally, the ": " is another literal that matches the end of the se-

quence. You can experiment with it freely before you commit to us-

ing a particular expression, by asking R to echo the results to the

command line, using the function str_match() like this:

str_match(modtext,"RT @[a-z,A-Z]*: ")

Once you are satisﬁed that this expression matches the retweet

phrases properly, you can commit the results to a new column/

ﬁeld/variable in the dataframe:

> tweetDF$rt <- str_match(modtext,"RT @[a-z,A-Z]*: ")

> detach(tweetDF)

> attach(tweetDF)

Now you can review what you found by echoing the new variable

"rt" to the command line or by examining it in R-studio’s data

browser:

> head(rt, 10)

[,1]

[1,] NA

[2,] NA

[3,] NA

[4,] NA

[5,] NA

[6,] NA

[7,] NA

[8,] "RT @SEIA: "

[9,] NA

[10,] "RT @andyschonberger: "

This may be the ﬁrst time we have seen the value "NA." In R, NA

means that there is no value available, in effect that the location is

empty. Statisticians also call this missing data. These NAs appear

in cases where there was no match to the regular expression that

we provided to the function str_match(). So there is nothing wrong

116

here, this is an expected outcome of the fact that not all tweets

were retweets. If you look carefully, though, you will see some-

thing else that is interesting.

R is trying to tell us something with the bracket notation. At the

top of the list there is a notation of [,1] which signiﬁes that R is

showing us the ﬁrst column of something. Then, each of the entries

looks like [#,] with a row number in place of # and an empty col-

umn designator, suggesting that R is showing us the contents of a

row, possibly across multiple columns. This seems a bit mysteri-

ous, but a check of the documentation for str_match() reveals that

it returns a matrix as its result. This means that tweetDF$rt could

potentially contain its own rectangular data object: In effect, the

variable rt could itself contain more than one column!

In our case, our regular expression is very simple and it contains

just one chunk to match, so there is only one column of new data

in tweetDF$rt that was generated form using str_match(). Yet the

full capability of regular expressions allows for matching a whole

sequence of chunks, not just one, and so str_match() has set up the

data that it returns to prepare for the eventuality that each row of

tweetDF$rt might actually have a whole list of results.

If, for some reason, we wanted to simplify the structure of

tweetDF$rt so that each element was simply a single string, we

could use this command:

tweetDF$rt <- tweetDF$rt[ ,1]

This assigns to each element of tweetDF$rt the contents of the ﬁrst

column of the matrix. If you run that command and reexamine

tweetDF$rt with head() you will ﬁnd the simpliﬁed structure: no

more column designator.

For us to be able to make some use of the retweet string we just iso-

lated, we probably should extract just the "screenname" of the indi-

vidual whose tweet got retweeted. A screenname in Twitter is like

a username, it provides a unique identiﬁer for each person who

wants to post tweets. An individual who is frequently retweeted

by others may be more inﬂuential because their postings reach a

wider audience, so it could be useful for us to have a listing of all

of the screennames without the extraneous stuff. This is easy to do

with str_replace(). Note that we used str_replace_all() earlier in the

chapter, but we don’t need it here, because we know that we are

going to replace just one instance of each string:

tweetDF$rt<-str_replace(rt, "RT @","")

tweetDF$rt<-str_replace(rt,": ","")

> tail(rt, 1)

[,1]

[100,] "SolarFred"

tweetDF$rt <- tweetDF$rt[ ,1]

In the ﬁrst command, we substitute the empty string in place of the

four character preﬁx "RT @", while in the second command we sub-

stitute the empty string in place of the two character sufﬁx ": ". In

each case we assign the resulting string back to tweetDF$rt. You

may be wondering why sometimes we create a new column or

ﬁeld when we calculate some new data while other times we do

not. The golden rule with data columns is never to mess with the

117

original data that was supplied. When you are working ona "de-

rived" column, i.e., one that is calculated from other data, it may

require several intermediate steps to get the data looking the way

you want. In this case, rt is a derived column that we extracted

from the text ﬁeld of the tweet and our goal was to reduce it to the

bare screenname of the individual whose post was retweeted. So

these commands, which successfully overwrite rt with closer and

closer versions of what we wanted, were fair game for modiﬁca-

tion.

You may also have noticed

the very last command. It

seems that one of our steps,

probably the use of

str_match() must have

"matrix-ized" our data

again, so we use the column

trick that appeared earlier in

this chapter to ﬂatten the ma-

trix back to a single column

of string data.

This would be a good point to visualize what we have obtained.

Here we introduce two new functions, one which should seem fa-

miliar and one that is quite new:

table(as.factor(rt))

The as.factor() function is a type/mode coercion and just a new

one in a family we have seen before. In previous chapters we used

as.integer() and as.character() to perform other conversions. In R a

factor is a collection of descriptive labels and corresponding

unique identifying numbers. The identifying numbers are not usu-

ally visible in outputs. Factors are often used for dividing up a data-

set into categories. In a survey, for instance, if you had a variable

containing the gender of a participant, the variable would fre-

quently be in the form of a factor with (at least) two distinct catego-

ries (or what statisticians call levels), male and female. Inside R,

each of these categories would be represented as a number, but the

corresponding label would usually be the only thing you would

see as output. As an experiment, try running this command:

str(as.factor(rt))

This will reveal the

"structure" of the data

object after coercion.

Returning to the earlier

table(as.factor(rt)) com-

mand, the table() func-

tion takes as input one

or more factors and re-

turns a so called contin-

gency table. This is easy to understand for use with just one factor:

The function returns a unique list of factor "levels" (unique: mean-

ing no duplicates) along with a count of how many rows/instances

there were of each level in the dataset as a whole.

The screen shot on this page shows the command and the output.

There are about 15 unique screennames of Twitter users who were

retweeted. The highest number of times that a screenname ap-

peared was three, in the case of SEIA. The table() function is used

more commonly to create two-way (two dimensional) contingency

118

tables. We could demonstrate that here if we had two factors, so

let’s create another factor.

Remember earlier in the chapter we noticed some tweets had texts

that were longer than 140 characters. We can make a new variable,

we’ll call it longtext, that will be TRUE if the original tweet was

longer than 140 characters and FALSE if it was not:

> tweetDF$longtext <- (textlen>140)

> detach(tweetDF)

> attach(tweetDF)

The ﬁrst command above has an inequality expression on the right

hand side. This is tested for each row and the result, either TRUE

or FALSE, is assigned to the new variable longtext. Computer scien-

tists sometimes call this a "ﬂag" variable because it ﬂags whether or

not a certain attribute is present in the data. Now we can run the

table() function on the two factors:

> table(as.factor(rt),as.factor(longtext))

FALSE TRUE

EarthTechling 0 1

FeedTheGrid 2 0

FirstSolar 1 0

GreenergyNews 1 0

RayGil 0 1

SEIA 3 0

SolarFred 0 2

SolarIndustry 1 0

SolarNovus 1 0

andyschonberger 0 2

deepgreendesign 0 1

gerdvdlogt 2 0

seia 2 0

solarfred 1 0

thesolsolution 1 0

For a two-way contingency table, the ﬁrst argument you supply to

table() is used to build up the rows and the second argument is

used to create the columns. The command and output above give

us a nice compact display of which retweets are longer than 140

characters (the TRUE column) and which are not (the FALSE col-

umn). It is easy to see at a glance that there are many in each cate-

gory. So, while doing a retweet may contribute to having an extra

long tweet, there are also many retweets that are 140 characters or

less. It seems a little cumbersome to look at the long list of retweet

screennames, so we will create another ﬂag variable that indicates

whether a tweet text contains a retweet. This will just provide a

more compact way of reviewing which tweets have retweets and

which do not:

> tweetDF$hasrt <- !(is.na(rt))

> detach(tweetDF)

119

> attach(tweetDF)

> View(tweetDF)

The ﬁrst command above uses a function we have not encountered

before: is.na(). A whole family of functions that start with "is" exists

in R (as well as in other programming languages) and these func-

tions provide a convenient way of testing the status or contents of

a data object or of a particular element of a data object. The is.na()

function tests whether an element of the input variable has the

value NA, which we know from earlier in the chapter is R’s way of

showing a missing value (when a particular data element is

empty). So the expression, is.na(rt) will return TRUE if a particular

cell of tweetDF$rt contains the empty value NA, and false if it con-

tains some real data. If you look at the name of our new variable,

however, which we have called "hasrt" you may see that we want

to reverse the sense of the TRUE and FALSE that is.na() returns. To

do that job we use the "!" character, which computers scientists

may either call "bang" or more accurately, "not." Using "not" is

more accurate because the "!" character provides the Boolean NOT

function, which changes a TRUE to a FALSE and vice versa. One

last little thing is that the View() command causes R-Studio to

freshen the display of the dataframe in its upper left hand pane.

Let’s look again at retweets and long tweet texts:

> table(hasrt,longtext)

longtext

hasrt FALSE TRUE

FALSE 76 2

TRUE 15 7

There are more than twice as many extra long texts (7) when a

tweet contains a retweet than when it does not.

Let’s now follow the same general procedure for extracting the

URLs from the tweet texts. As before the goal is to create a new

string variable/column on the original dataframe that will contain

the URLs for all of those tweets that have them. Additionally, we

will create a ﬂag variable that signiﬁes whether or not each tweet

contains a URL. Here, as before, we follow a key principle: Don’t

mess with your original data. We will need to develop a new regu-

lar expression in order to locate an extract the URL string from in-

side of the tweet text. Actually, if you examine your tweet data in

the R-Studio data browser, you may note that some of the tweets

have more than one URL in them. So we will have to choose our

function call carefully and be equally careful looking at the results

to make sure that we have obtained what we need.

At the time when this was written, Twitter had imposed an excel-

lent degree of consistency on URLs, such that they all seem to start

with the string "http://t.co/". Additionally, it seems that the com-

pacted URLs all contain exactly 8 characters after that literal, com-

posed of upper and lower case letters and digits. We can use

str_match_all() to extract these URLs using the following code:

str_match_all(text,"http://t.co/[a-z,A-Z,0-9]{8}")

We feed the tweetDF$text ﬁeld as input into this function call (we

don’t need to provide the tweetDF$ part because this dataframe is

attached). The regular expression begins with the 12 literal charac-

ters ending with a forward slash. Then we have a regular expres-

sion pattern to match. The material within the square brackets

matches any upper or lowercase letter and any digit. The numeral

120

8 between the curly braces at the end say to match the previous pat-

tern exactly eight times. This yields output that looks like this:

[[6]]

[,1]

[1,] "http://t.co/w74X9jci"

[[7]]

[,1]

[1,] "http://t.co/DZBUoz5L"

[2,] "http://t.co/gmtEdcQI"

This is just an excerpt of the output, but there are a couple of impor-

tant things to note. First, note that the ﬁrst element is preceded by

the notation [[6]]. In the past when R has listed out multiple items

on the output, we have seen them with index numbers like [1] and

[2]. In this case, however, that could be confusing because each ele-

ment in the output could have multiple rows (as item [[7]] above

clearly shows). So R is using double bracket notation to indicate

the ordinal number of each chunk of data in the list, where a given

chunk may itself contain multiple elements.

Confusing? Let’s go at it from a different angle. Look at the output

under the [[7]] above. As we noted a few paragraphs ago, some of

those tweets have multiple URLs in them. The str_match_all() func-

tion handles this by creating, for every single row in the tweet data, a

data object that itself contains exactly one column but one or possi-

bly more than one row - one row for each URL that appears in the

tweet. So, just as we saw earlier in the chapter, we are getting back

from a string function a complex matrix-like data object that re-

quires careful handling if we are to make proper use of it.

The only other bit of complexity is this: What if a tweet contained

no URLs at all? Your output from running the str_match_all() func-

tion probably contains a few elements that look like this:

[[30]]

character(0)

[[31]]

character(0)

So elements [[30]] and [[31]] of the data returned from

str_match_all() each contain a zero length string. No rows, no col-

umns, just character(0), the so-called null character, which in many

computer programming languages is used to "terminate" a string.

Let’s go ahead and store the output from str_match_all() into a

new vector on tweetDF and then see what we can do to tally up

the URLs we have found:

> tweetDF$urlist<-str_match_all(text,+"

"http://t.co/[a-z,A-Z,0-9]{8}")

> detach(tweetDF)

> attach(tweetDF)

> head(tweetDF$urlist,2)

[[1]]

121

[,1]

[1,] "http://t.co/ims8gDWW"

[[2]]

[,1]

[1,] "http://t.co/37PKAF3N"

Now we are ready to wrestle with the problem of how to tally up

the results of our URL parsing. Unlike the situation with retweets,

where there either was or was not a single retweet indication in the

text, we have the possibility of zero, one or more URLs within the

text of each tweet. Our new object "urlist" is a multi-dimensional

object that contains a single null character, one row/column of

character data, or one column with more than one row of character

data. The key to summarizing this is the length() function, which

will happily count up the number of elements in an object that you

supply to it:

> length(urlist[[1]])

[1] 1

> length(urlist[[5]])

[1] 0

> length(urlist[[7]])

[1] 2

Here you see that double bracket notation again, used as an index

into each "chunk" of data, where the chunk itself may have some

internal complexity. In the case of element [[1]] above, there is one

row, and therefore one URL. For element [[5]] above, we see a zero,

which means that length() is telling us that this element has no

rows in it at all. Finally, for element [[7]] we see 2, meaning that

this element contains two rows, and therefore two URLs.

In previous work with R, we’ve gotten used to leaving the inside

of the square brackets empty when we want to work with a whole

list of items, but that won’t work with the double brackets:

> length(urlist[[]])

Error in urlist[[]] : invalid subscript type 'symbol'

The double brackets notation is designed to reference just a single

element or component in a list, so empty double brackets does not

work as a shorthand for every element in a list. So what we must

do if we want to apply the length() function to each element in url-

ist is to loop. We could accomplish this with a for loop, as we did

in the last chapter, using an index quantity such as "i" and substitut-

ing i into each expression like this: urlist[[i]]. But let’s take this op-

portunity to learn a new function in R, one that is generally more

efﬁcient for looping. The rapply() function is part of the "apply"

family of functions, and it stands for "recursive apply." Recursive

in this case means that the function will dive down into the com-

plex, nested structure of urlist and repetitively run a function for

us, in this case the length() function:

> tweetDF$numurls<-rapply(urlist,length)

> detach(tweetDF)

> attach(tweetDF)

> head(numurls,10)

122

[1] 1 1 1 1 0 1 2 1 1 1

Excellent! We now have a new ﬁeld on tweetDF that counts up the

number of URLs. As a last step in examining our tweet data, let’s

look at a contingency table that looks at the number of URLs to-

gether with the ﬂag indicating an extra long tweet. Earlier in the

chapter, we mentioned that the table() function takes factors as its

input. In the command below we have supplied the numurls ﬁeld

to the table() function without coercing it to a factor. Fortunately,

the table() function has some built in intelligence that will coerce a

numeric variable into a factor. In this case because numurls only

takes on the values of 0, 1, or 2, it makes good sense to allow ta-

ble() to perform this coercion:

> table(numurls,longtext)

longtext

numurls FALSE TRUE

0 16 3

1 72 6

2 3 0

This table might be even more informative if we looked at it as pro-

portions, so here is a trick to view proportions instead of counts:

> prop.table(table(numurls,longtext))

longtext

numurls FALSE TRUE

0 0.16 0.03

1 0.72 0.06

2 0.03 0.00

That looks familiar! Now, of course, we remember that we had ex-

actly 100 tweets, so each of the counts could be considered a per-

centage with no further calculation. Still, prop.table() is a useful

function to have when you would rather view your contingency

tables as percentages rather than counts. We can see from these re-

sults that six percent of the tweets have one URL, but only three

percent have no URLS.

So, before we close out this chapter, let’s look at a three way contin-

gency table by putting together our two ﬂag variables and the num-

ber of URLs:

> table(numurls,hasrt,longtext)

, , longtext = FALSE

hasrt

numurls FALSE TRUE

0 15 1

1 58 14

2 3 0

, , longtext = TRUE

hasrt

numurls FALSE TRUE

0 0 3

123

1 2 4

2 0 0

Not sure this entirely solves the mystery, but if we look at the sec-

ond two-way table above, where longtext = TRUE, it seems that ex-

tra long tweets either have a retweet (3 cases), or a single URL (2

cases) or both (4 cases).

When we said we would give statistics a little rest in this chapter,

we lied just a tiny bit. Check out these results:

> mean(textlen[hasrt&longtext])

[1] 155

> mean(textlen[!hasrt&longtext])

[1] 142

In both commands we have requested the mean of the variable

textlen, which contains the length of the original tweet (the one

without the space stripped out). In each command we have also

used the bracket notation to choose a particular subset of the cases.

Inside the brackets we have a logical expression. The only cases

that will be included in the calculation of the mean are those where

the expression inside the brackets evaluates to TRUE. In the ﬁrst

command we ask for the mean tweet length for those tweets that

have a retweet AND are extra long (the ampersand is the Boolean

AND operator). In the second command we use the logical NOT

(the "!" character) to look at only those cases that have extra long

text but do not have a retweet. The results are instructive. The

really long tweets, with a mean length of 155 characters, are those

that have retweets. It seems that Twitter does not penalize an indi-

vidual who retweets by counting the number of characters in the

"RT @SCREENNAME:" string. If you have tried the web interface

for Twitter you will see why this makes sense: Retweeting is accom-

plished with a click, and the original tweet - which after all may al-

ready be 140 characters - appears underneath the screenname of

the originator of the tweet. The "RT @" string does not even appear

in the text of the tweet at that point.

Looking back over this chapter, we took a close look at some of the

string manipulation functions provided by the package "stringr".

These included some of the most commonly used actions such as

ﬁnding the length of a string, ﬁnding matching text within a string,

and doing search and replace operations on a string. We also be-

came aware of some additional complexity in nested data struc-

tures. Although statisticians like to work with nice, well-ordered

rectangular datasets, computer scientists often deal with much

more complex data structures - although these are built up out of

parts that we are familiar with such as lists, vectors, and matrices.

Twitter is an excellent source of string data, and although we have

not yet done much in analyzing the contents of tweets or their

meanings, we have looked at some of the basic features and regu-

larities of the text portion of a tweet. In the next chapter we will be-

come familiar with a few additional text tools and then be in a posi-

tion to manipulate and analyze text data

Chapter Challenges

Create a function that takes as input a dataframe of tweets and re-

turns as output a list of all of the retweet screennames. As an extra

challenge, see if you can reduce that list of screennames to a

unique set (i.e., no duplicates) while also generating a count of the

number of times that each retweet screenname appeared.

124

Once you have written that function, it should be a simple matter

to copy and modify it to create a new function that extracts a

unique list of hashtags from a dataframe of tweets. Recall that

hashtags begin with the "#" character and may contain any combi-

nation of upper and lowercase characters as well as digits. There is

no length limit on hashtags, so you will have to assume that a hash-

tag ends when there is a space or a punctuation mark such as a

comma, semicolon, or period.

Sources

http://cran.r-project.org/web/packages/stringr/index.html

http://en.wikipedia.org/wiki/ASCII

http://en.wikipedia.org/wiki/Regular_expression

http://en.wikipedia.org/wiki/Unicode

http://had.co.nz/ (Hadley Wickham)

http://mashable.com/2010/08/14/twitter-140-bug/

http://stat.ethz.ch/R-manual/R-devel/library/base/html/search

.html

http://stat.ethz.ch/R-manual/R-devel/library/base/html/table.h

tml

R Code for TweetFrame() Function

# TweetFrame() - Return a dataframe based on a search of Twit-

ter

TweetFrame<-function(searchTerm, maxTweets)

{

tweetList <- searchTwitter(searchTerm, n=maxTweets)

# as.data.frame() coerces each list element into a row

# lapply() applies this to all of the elements in twtList

# rbind() takes all of the rows and puts them together

# do.call() gives rbind() all rows as individual elements

tweetDF<- do.call("rbind", lapply(tweetList,as.data.frame))

# This last step sorts the tweets in arrival order

return(tweetDF[order(as.integer(tweetDF$created)), ])

}

125

In the previous chapter we mastered some of the most basic and important functions for examining

and manipulating text. Now we are in a position to analyze the actual words that appear in text

documents. Some of the most basic functions of the Internet, such as keyword search, are

accomplished by analyzing the "content" i.e., the words in a body of text.

CHAPTER 13

126

Word Perfect

The picture at the start of this chapter is a so called "word cloud"

that was generated by examining all of the words returned from a

Twitter search of the term "data science" (using a web application

at http://www.jasondavies.com) These colorful word clouds are

fun to look at, but they also do contain some useful information.

The geometric arrangement of words on the ﬁgure is partly ran-

dom and partly designed and organized to please the eye. Same

with the colors. The font size of each word, however, conveys some

measure of its importance in the "corpus" of words that was pre-

sented to the word cloud graphics program. Corpus, from the

Latin word meaning "body," is a word that text analysts use to refer

to a body of text material, often consisting of one or more docu-

ments. When thinking about a corpus of textual data, a set of docu-

ments could really be anything: web pages, word processing docu-

ments on your computer, a set of Tweets, or government reports. In

most cases, text analysts think of a collection of documents, each of

which contains some natural language text, as a corpus if they plan

to analyze all the documents together.

The word cloud on the previous page shows that "Data" and "Sci-

ence" are certainly important terms that came from the search of

Twitter, but there are dozens and dozens of less important, but per-

haps equally interesting, words that the search results contained.

We see words like algorithms, molecules, structures, and research,

all of which could make sense in the context of data science. We

also see other terms, like #christian, Facilitating, and Coordinator,

that don’t seem to have the same obvious connection to our origi-

nal search term "data science." This small example shows one of

the fundamental challenges of natural language processing and the

closely related area of search: ensuring that the analysis of text pro-

duces results that are relevant to the task that the user has in mind.

In this chapter we will use some new R packages to extend our

abilities to work with text and to build our own word cloud from

data retrieved from Twitter. If you have not worked on the chapter

"String Theory" that precedes this chapter, you should probably do

so before continuing, as we build on the skills developed there.

Depending upon where you left off after the previous chapter, you

will need to retrieve and pre-process a set of tweets, using some of

the code you already developed, as well as some new code. At the

end of the previous chapter, we have provided sample code for the

TweetFrame() function, that takes a search term and a maximum

tweet limit and returns a time-sorted dataframe containing tweets.

Although there are a number of comments in that code, there are

really only three lines of functional code thanks to the power of the

twitteR package to retrieve data from Twitter for us. For the activi-

ties below, we are still working with the dataframe that we re-

trieved in the previous chapter using this command:

tweetDF <- TweetFrame("#solar",100)

This yields a dataframe, tweetDF, that contains 100 tweets with the

hashtag #solar, presumably mostly about solar energy and related

"green" topics. Before beginning our work with the two new R

packages, we can improve the quality of our display by taking out

a lot of the junk that won’t make sense to show in the word cloud.

To accomplish this, we have authored another function that strips

out extra spaces, gets rid of all URL strings, takes out the retweet

header if one exists in the tweet, removes hashtags, and eliminates

references to other people’s tweet handles. For all of these transfor-

mations, we have used string replacement functions from the

stringr package that was introduced in the previous chapter. As an

example of one of these transformations, consider this command,

127

which appears as the second to last line of the CleanTweet() func-

tion:

tweets <- str_replace_all(tweets,"@[a-z,A-Z]*","")

You should feel pretty comfortable reading this line of code, but if

not, here’s a little more practice. The left hand side is easy: we use

the assignment arrow to assign the results of the right hand side

expression to a data object called "tweets." Note that when this

statement is used inside the function as shown at the end of the

chapter, "tweets" is a temporary data object, that is used just within

CleanTweets() after which it disappears automatically.

The right hand side of the expression uses the str_replace_all() func-

tion from the stringr package. We use the "all" function rather than

str_replace() because we are expecting multiple matches within

each individual tweet. There are three arguments to the str_re-

place_all() function. The ﬁrst is the input, which is a vector of char-

acter strings (we are using the temporary data object "tweets" as

the source of the text data as well as its destination), the second is

the regular expression to match, and the third is the string to use to

replace the matches, in this case the empty string as signiﬁed by

two double quotes with nothing between them. The regular expres-

sion in this case is the at sign, "@", followed by zero or more upper

and lowercase letters. The asterisk, "*", after the stuff in the square

brackets is what indicates the zero or more. That regular expres-

sion will match any screenname referral that appears within a

tweet.

If you look at a few tweets you will ﬁnd that people refer to each

other quite frequently by their screennames within a tweet, so @So-

larFred might occur from time to time within the text of a tweet.

Here’s something you could investigate on your own: Can screen-

names contain digits as well as letters? If so, how would you have

to change the regular expression in order to also match the digits

zero through nine as part of the screenname? On a related note,

why did we choose to strip these screen names out of our tweets?

What would the word cloud look like if you left these screennames

in the text data?

Whether you typed in the function at the end of this chapter or you

plan to enter each of the cleaning commands individually, let’s be-

gin by obtaining a separate vector of texts that is outside the origi-

nal dataframe:

> cleanText<-tweetDF$text

> head(cleanText, 10)

There’s no critical reason for doing this except that it will simplify

the rest of the presentation. You could easily copy the tweetDF$text

data into another column in the same dataframe if you wanted to.

We’ll keep it separate for this exercise so that we don’t have to

worry about messing around with the rest of the dataframe. The

head() command above will give you a preview of what you are

starting with. Now let’s run our custom cleaning function:

> cleanText<-CleanTweets(cleanText)

> head(cleanText, 10)

Note that we used our "cleanText" data object in the ﬁrst command

above as both the source and the destination. This is an old com-

puter science trick for cutting down on the number of temporary

variables that need to be used. In this case it will do exactly what

we want, ﬁrst evaluating the right hand side of the expression by

128

running our CleanTweets() function with the cleanText object as in-

put and then taking the result that is returned by CleanTweets()

and assigning it back into cleanText, thus overwriting the data that

was in there originally. Remember that we have license to do what-

ever we want to cleanText because it is a copy of our original data,

and we have left the original data intact (i.e., the text column inside

the tweetDF dataframe).

The head() command should now show a short list of tweets with

much of the extraneous junk ﬁltered out. If you have followed

these steps, cleanText is now a vector of character strings (in this

example exactly 100 strings) ready for use in the rest of our work

below. We will now use the "tm" package to process our texts. The

"tm" in this case refers to "text mining," and is a popular choice

among the many text analysis packages available in R. By the way,

text mining refers to the practice of extracting useful analytic infor-

mation from corpora of text (corpora is the plural of corpus). Al-

though some people use text mining and natural language process-

ing interchangeably, there are probably a couple subtle differences

worth considering. First, the "mining" part of text mining refers to

an area of practice that looks for unexpected patterns in large data

sets, or what some people refer to as knowledge discovery in data-

bases. In contrast, natural language processing reﬂects a more gen-

eral interest in understanding how machines can be programmed

(or learn on their own) how to digest and make sense of human lan-

guage. In a similar vein, text mining often focuses on statistical ap-

proaches to analyzing text data, using strategies such as counting

word frequencies in a corpus. In natural language processing, one

is more likely to hear consideration given to linguistics, and there-

fore to the processes of breaking text into its component grammati-

cal pieces such as nouns and verbs. In the case of the "tm" add on

package for R, we are deﬁnitely in the statistical camp, where the

main process is to break down a corpus into sequences of words

and then to tally up the different words and sequences we have

found.

To begin, make sure that the tm package is installed and "library-

ed" in your copy of R and R-Studio. You can use the graphic inter-

face in R-Studio for this purpose or the EnsurePackage() function

that we wrote in a previous chapter. Once the tm package is ready

to use, you should be able to run these commands:

> tweetCorpus<-Corpus(VectorSource(cleanText))

> tweetCorpus

A corpus with 100 text documents

> tweetCorpus<-tm_map(tweetCorpus, tolower)

> tweetCorpus<-tm_map(tweetCorpus, removePunctuation)

> tweetCorpus<-tm_map(tweetCorpus,removeWords,+"

stopwords('english'))

In the ﬁrst step above , we "coerce" our cleanText vector into a cus-

tom "Class" provided by the tm package and called a "Corpus,"

storing the result in a new data object called "tweetCorpus." This is

the ﬁrst time we have directly encountered a "Class." The term

"class" comes from an area of computer science called "object ori-

ented programming." Although R is different in many ways from

object-oriented languages such as Java, it does contain many of the

most fundamental features that deﬁne an object oriented language.

For our purposes here, there are just a few things to know about a

class. First, a class is nothing more or less than a deﬁnition for the

structure of a data object. Second, classes use basic data types, such

129

as numbers, to build up more complex data structures. For exam-

ple, if we made up a new "Dashboard" class, it could contain one

number for "Miles Per Hour," another number for "RPM," and per-

haps a third one indicating the remaining "Fuel Level." That brings

up another point about Classes: users of R can build their own. In

this case, the author of the tm package, Ingo Feinerer, created a

new class, called Corpus, as the central data structure for text min-

ing functions. (Feinerer is a computer science professor who works

at the Vienna University of Technology in the Database and Artiﬁ-

cial Intelligence Group.) Last, and most important for this discus-

sion, a Class not only contains deﬁnitions about the structure of

data, it also contains references to functions that can work on that

Class. In other words, a Class is a data object that carries with it in-

structions on how to do operations on it, from simple things like

add and subtract all the way up to complicated operations such as

graphing.

In the case of the tm package, the Corpus Class deﬁnes the most

fundamental object that text miners care about, a corpus contain-

ing a collection of documents. Once we have our texts stored in a

Corpus, the many functions that the tm package provides to us are

available. The last three commands in the group above show the

use of the tm_map() function, which is one of the powerful capabili-

ties provided by tm. In each case where we call the tm_map() func-

tion, we are providing tweetCorpus as the input data, and then we

are providing a command that undertakes a transformation on the

corpus. We have done three transformations here, ﬁrst making all

of the letters lowercase, then removing the punctuation, and ﬁnally

taking out the so called "stop" words.

The stop words deserve a little explanation. Researchers who devel-

oped the early search engines for electronic databases found that

certain words interfered with how well their search algorithms

worked. Words such as "the," "a," and "at" appeared so commonly

in so many different parts of the text that they were useless for dif-

ferentiating between documents. The unique and unusual nouns,

verbs, and adjectives that appeared in a document did a much bet-

ter job of setting a document apart from other documents in a cor-

pus, such that researchers decided that they should ﬁlter out all of

the short, commonly used words. The term "stop words" seems to

have originated in the 1960s to signify words that a computer proc-

essing system would throw out or "stop using" because they had

little meaning in a data processing task. To simplify the removal of

stop words, the tm package contains lists of such words for differ-

ent languages. In the last command on the previous page we re-

quested the removal of all of the common stop words.

At this point we have processed our corpus into a nice uniform

"bag of words" that contains no capital letters, punctuation, or stop

words. We are now ready to conduct a kind of statistical analysis of

the corpus by creating what is known as a "term-document ma-

trix." The following command from the tm package creates the ma-

trix:

> tweetTDM<-TermDocumentMatrix(tweetCorpus)

> tweetTDM

A term-document matrix (375 terms, 100 documents)

Non-/sparse entries: 610/36890

Sparsity : 98%

130

Maximal term length: 21

Weighting : term frequency (tf)

A term-document matrix, also sometimes called a document-term

matrix, is a rectangular data structure with terms as the rows and

documents as the columns (in other uses you may also make the

terms as columns and documents as rows). A term may be a single

word, for example, "biology," or it could also be a compound word,

such as "data analysis." The process of determining whether words

go together in a compound word can be accomplished statistically

by seeing which words commonly go together, or it can be done

with a dictionary. The tm package supports the dictionary ap-

proach, but we have not used a dictionary in this example. So if a

term like "data" appears once in the ﬁrst document, twice in the sec-

ond document, and not at all in the third document, then the col-

umn for the term data will contain 1, 2, 0.

The statistics reported when we ask for tweetTDM on the com-

mand line give us an overview of the results. The TermDocument-

Matrix() function extracted 375 different terms from the 100 tweets.

The resulting matrix mainly consists of zeros: Out of 37,500 cells in

the matrix, only 610 contain non-zero entries, while 36,890 contain

zeros. A zero in a cell means that that particular term did not ap-

pear in that particular document. The maximal term length was 21

words, which an inspection of the input tweets indicates is also the

maximum word length of the input tweets. Finally, the last line,

starting with "Weighting" indicates what kind of statistic was

stored in the term-document matrix. In this case we used the de-

fault, and simplest, option which simply records the count of the

number of times a term appears across all of the documents in the

corpus. You can peek at what the term-document matrix contains

by using the inspect function:

inspect(tweetTDM)

Be prepared for a large amount of output. Remember the term

"sparse" in the summary of the matrix? Sparse refers to the over-

whelming number of cells that contain zero - indicating that the

particular term does not appear in a given document. Most term

document matrices are quite sparse. This one is 98% sparse be-

cause 36890/37500 = 0.98. In most cases we will need to cull or ﬁl-

ter the term-document matrix for purposes of presenting or visual-

izing it. The tm package provides several methods for ﬁltering out

sparsely used terms, but in this example we are going to leave the

heavy lifting to the word cloud package.

As a ﬁrst step we need to install and library() the "wordcloud"

package. As with other packages, either use the package interface

in R-Studio or the EnsurePackage() function that we wrote a few

chapters ago. The wordcloud package was written by freelance stat-

istician Ian Fellows, who also developed the "Deducer" user inter-

face for R. Deducer provides a graphical interface that allows users

who are more familiar with SPSS or SAS menu systems to be able

to use R without resorting to the command line.

Once the wordcloud package is loaded, we need to do a little

preparation to get our data ready to submit to the word cloud gen-

erator function. That function expects two vectors as input argu-

ments, the ﬁrst a list of the terms, and the second a list of the fre-

quencies of occurrence of the terms. The list of terms and frequen-

cies must be sorted with the most frequent terms appearing ﬁrst.

To accomplish this we ﬁrst have to coerce our tweet data back into

131

a plain data matrix so that we can sort it by frequency. The ﬁrst

command below accomplishes this:

> tdMatrix <- as.matrix(tweetTDM)

> sortedMatrix<-sort(rowSums(tdMatrix),+"

decreasing=TRUE)

> cloudFrame<-data.frame( +"

word=names(sortedMatrix),freq=sortedMatrix)

> wordcloud(cloudFrame$word,cloudFrame$freq)

In the next command above, we are accomplishing two things in

one command: We are calculating the sums across each row, which

gives us the total frequency of a term across all of the different

tweets/documents. We are also sorting the resulting values with

the highest frequencies ﬁrst. The result is a named list: Each item of

the list has a frequency and the name of each item is the term to

which that frequency applies.

In the second to last command above, we are extracting the names

from the named list in the previous command and binding them

together into a dataframe with the frequencies. This dataframe,

"cloudFrame", contains exactly the same information as the named

list. "sortedMatrix," but cloudFrame has the names in a separate

column of data. This makes it easier to do the ﬁnal command

above, which is the call to the wordcloud() function. The word-

cloud() function has lots of optional parameters for making the

word cloud more colorful, controlling its shape, and controlling

how frequent an item must be before it appears in the cloud, but

we have used the default settings for all of these parameters for the

sake of simplicity. We pass to the wordcloud() function the term

list and frequency list that we bound into the dataframe and word-

cloud() produces the nice graphic that you see below.

If you recall the Twitter search that we used to retrieve those

tweets (#solar) it makes perfect sense that "solar" is the most fre-

quent term (even though we ﬁltered out all of the hashtags. The

next most popular term is "energy" and after that there are a vari-

ety of related words such as "independence," "green," "wind," and

"metering."

132

Chapter Challenge

Develop a function that builds upon previous functions we have

developed, such as TweetFrame() and CleanTweets(), to take a

search term, conduct a Twitter search, clean up the resulting texts,

formulate a term-document matrix, and submit resulting term fre-

quencies to the wordcloud() function. Basically this would be a

"turnkey" package that would take a Twitter search term and pro-

duce a word cloud from it, much like the Jason Davies site de-

scribed at the beginning of this chapter.

Sources Used in This Chapter

http://cran.r-project.org/web/packages/wordcloud/wordcloud.

pdf

http://www.dbai.tuwien.ac.at/staff/feinerer/

http://en.wikipedia.org/wiki/Document-term_matrix

http://en.wikipedia.org/wiki/Stop_words

http://en.wikipedia.org/wiki/Text_mining

http://stat.ethz.ch/R-manual/R-devel/library/base/html/colSu

ms.html

http://www.jasondavies.com/wordcloud/

R Code for CleanTweets() Function

# CleanTweets() - Takes the junk out of a vector of

# tweet texts

CleanTweets<-function(tweets)

{

# Remove redundant spaces

tweets <- str_replace_all(tweets," "," ")

# Get rid of URLs

tweets <- str_replace_all(tweets, + "

"http://t.co/[a-z,A-Z,0-9]{8}","")

# Take out retweet header, there is only one

tweets <- str_replace(tweets,"RT @[a-z,A-Z]*: ","")

# Get rid of hashtags

tweets <- str_replace_all(tweets,"#[a-z,A-Z]*","")

# Get rid of references to other screennames

tweets <- str_replace_all(tweets,"@[a-z,A-Z]*","")

return(tweets)

}

133

Before now we have only used small amount of data that we typed in ourselves, or somewhat larger

amounts that we extracted from Twitter. The world is full of other sources of data, however, and we

need to examine how to get them into R, or at least how to make them accessible for manipulation in

R. In this chapter, we examine various ways that data are stored, and how to access them.

CHAPTER 14

134

Storage Wars

Most people who have watched the evolution of technology over

recent decades remember a time when storage was expensive and

it had to be hoarded like gold. Over the last few years, however,

the accelerating trend of Moore’s Law has made data storage al-

most "too cheap to meter" (as they used to predict about nuclear

power). Although this opens many opportunities, it also means

that people keep data around for a long time, since it doesn’t make

sense to delete anything, and they may keep data around in many

different formats. As a result, the world is full of different data for-

mats, some of which are proprietary - designed and owned by a

single company such as SAS - and some of which are open, such as

the lowly but inﬁnitely useful "comma separated variable," or CSV

format.

In fact, one of the basic dividing lines in data formats is whether

data are human readable or not. Formats that are not human read-

able, often called binary formats, are very efﬁcient in terms of how

much data they can pack in per kilobyte, but are also squirrelly in

the sense that it is hard to see what is going on inside of the format.

As you might expect, human readable formats are inefﬁcient from

a storage standpoint, but easy to diagnose when something goes

wrong. For high volume applications, such as credit card process-

ing, the data that is exchanged between systems is almost univer-

sally in binary formats. When a data set is archived for later reuse,

for example in the case of government data sets available to the

public, they are usually available in multiple formats, at least one

of which is a human readable format.

Another dividing line, as mentioned above is between proprietary

and open formats. One of the most common ways of storing and

sharing small datasets is as Microsoft Excel spreadsheets. Although

this is a proprietary format, owned by Microsoft, it has also be-

come a kind of informal and ubiquitous standard. Dozens of differ-

ent software applications can read Excel formats (there are several

different formats that match different versions of Excel). In con-

trast, the OpenDocument format is an open format, managed by a

standards consortium, that anyone can use without worrying what

the owner might do. OpenDocument format is based on XML,

which stands for Extensible markup language. XML is a whole

topic in and of itself, but brieﬂy it is a data exchange format de-

signed speciﬁcally to work on the Internet and is both human and

machine readable. XML is managed by the W3C consortium,

which is responsible for developing and maintaining the many

standards and protocols that support the web.

As an open source program with many contributors, R offers a

wide variety of methods of connecting with external data sources.

This is both a blessing and a curse. There is a solution to almost

any data access problem you can imagine with R, but there is also

a dizzying array of options available such that it is not always obvi-

ous what to choose. We’ll tackle this problem in two different

ways. In the ﬁrst half of this chapter we will look at methods for

importing existing datasets. These may exist on a local computer

or on the Internet but the characteristic they share in common is

that they are contained (usually) within one single ﬁle. The main

trick here is to choose the right command to import that data into

R. In the second half of the chapter, we will consider a different

strategy, namely linking to a "source" of data that is not a ﬁle.

Many data sources, particularly databases, exist not as a single dis-

crete ﬁle, but rather as a system. The system provides methods or

calls to "query" data from the system, but from the perspective of

the user (and of R) the data never really take the form of a ﬁle.

135

The ﬁrst and easiest strategy for getting data into R is to use the

data import dialog in R-Studio. In the upper right hand pane of R-

Studio, the "Workspace" tab gives views of currently available data

objects, but also has a set of buttons at the top for managing the

work space. One of the cho