Cover Page



Title Page







Being “Literate”

It’s What They Think That Matters

What You Were Never Taught

Some Notes about Reading This Book


CHAPTER 1: Numerals Matter

Lining Up the Numbers

The Units of Measure


Negative Numbers: What Do They Mean?

CHAPTER 2: Looks Matter

The Tabula Rasa Decision

The “Where’s Waldo?” Effect

White Space Is Your Friend

…But White Space Is Not Always Your Friend

Time and Other Dimensions: Across or Down?

Borders, Shading, and Other Visual Effects

Comments and Artwork

The “Ransom Note” Effect

Two Truly Helpful Hints

CHAPTER 3: Words Matter

Mean What You Say and Say What You Mean

Choosing between Precision and Presentability

Treating Words Like Numbers

Remember the Title. Please.

CHAPTER 4: Your Audience Matters

The Right Amount of Data

Appropriate Emphasis on Critical Information


Meaningful and Relevant Numbers

Respect for your Audience’s Time


CHAPTER 5: You Can Pay Me Now . . .

The Instant Payoff Tips

Instant Payoff Tip #1: Set Up Templates and Styles

Instant Payoff Tip #2: Customize Your Toolbar

Instant Payoff Tip #3: Learn Some Shortcut Keys

Instant Payoff Tip #4: Use Consistent Formats

Instant Payoff Tip #5: Learn to Use Excel as a Database

Instant Payoff Tip #6: Learn to Use the Lookup & Reference Functions

Instant Payoff Tip #7: Organize Data for Easy Computation

Instant Payoff Tip #8: Learn to Use Automated Help

Instant Payoff Tip #9: Don’t Learn Too Many Ways to Do the Same Thing

CHAPTER 6: . . . Or Pay Me Later

The Long-Term Payoff Tips

Long-Term Payoff Tip #1: Use Sensible, Intelligent Filenames and Folder Organization

Long-Term Payoff Tip #2: Use Consistent Formats

Long-Term Payoff Tip #3: Design Intelligent, Intelligible Formulas

Long-Term Payoff Tip #4: Use Named Ranges

Long-Term Payoff Tip #5: Use Named Formulas or Macros

Long-Term Payoff Tip #6: Links: A Force That Can Be Used for Good or Evil

Long-Term Payoff Tip #7: Make Different Types of Cells Visually Distinguishable and Physically Separate

Long-Term Payoff Tip #8: Document Your Work!

Long-Term Payoff Tip #9: Check Your Work!

Long-Term Payoff Tip #10: Avoid Cool New Features

CHAPTER 7: Graphs: The “Cartoons” of Numbers

Why Do People Use Graphs?

Help Your Audience

First, Do No Harm

CHAPTER 8: The Pitfalls of Presentations and PowerPoint

Why Do People Make Fun of Business Presentations?

Real Estate Is a Scarce and Precious Commodity

Help Your Audience

First, Do No Harm

Some Basic Truths That Go Double for Quantation


CHAPTER 9: It’s Clear, But Is It Meaningful?

The War of the Adjectives

A Quantation Professional

Relating to Your Audience in a Constructive Way

CHAPTER 10: 53 . . . Uh, Is That a Lot?

What Is a Key Indicator?

What Makes a Good Key Indicator?

A Simple Example

How Do You Present Key Indicators Effectively?

A Note on Precision

A Note on Dashboards

CHAPTER 11: The One Report Every Organization Needs

A Sample Natural P&L, and What Makes It a Good One

1. One Page!

2. Decision-Focused Line Items

3. Appropriate Dollar Amounts, Neither Too Big Nor Too Small

4. Intuitive Organization of the Line Items

5. Understandable Categories, Meaningful to All Users

6. Plain-English Terminology

7. Consistent Look-and-Feel

8. Key Results Equal to the Corresponding Numbers in the Accounting System (or an Explanation Why Not)

Mapping the Chart of Accounts

Generate a Natural P&L from a Spreadsheet, or from the Accounting System?

Now Let’s Talk about the Columns

APPENDIX 11 A: A Sermon to the Accounting Purists

CHAPTER 12: The Gaps in GAAP

Rules-Based versus Principles-Based

Where Is All This Headed?

“Expensing” Stock Options

Software Revenue Recognition

Tying GAAP to Internal Management Reports

APPENDIX 12 A: Some Notes on GAAP

What Is GAAP?

Stock Options

Revenue Recognition

CHAPTER 13: Quantation: It’s Not Just for Business Anymore

One Taxpayer at a Time

All the Taxpayers at Once

What’s Really Going on Here?

CHAPTER 14: Quantation in Ordinary Life


The Meaning of Words

That Quiz I Promised


CHAPTER 15: Speaking Truth to Power

My “Tell the Truth” Syllogism

Reason #1 for Telling the Truth: It’s the Right Thing to Do

Reason #2: It’s in Your Employer’s Interest

Reason #3: It’s in Your Interest

Reason #4: You Will Get Caught

Reason #5: It’s the “Gotcha” of This Book

Don’t Be So Smug, You Civil Servants and Elected Officials!

Your Audience Has a Role, Too

CHAPTER 16: Now, What’s the First Thing You Do?

My Last, Best Tip

It’s Just a Skill

Where We Have Been

APPENDIX: Jazz Meets Theology


The Sins of Presentation

The Sins of Behavior

The Deadliest Sin of All

About the Author


Title Page

For Rachel and Molly, my favorite girls and my two favorite writers


I first met Randall Bolten in early 2009, as I was beginning my run for California statewide office. He was my contact for a dinner speech I was giving, and I sent him some economic data to be turned into slides and a handout. What came back was not just a correct and accurate presentation of the data, but one that was clear, concise, and comprehensible. Based on that experience, I retained Randall to prepare the economic and fiscal policy handouts that I used throughout the campaign. My data and graphical presentations became a mainstay of how I presented my public policy suggestions. I feel strongly that clearly and honestly presented data is essential to an informed electorate, and Randall made it possible for me to put that belief into practice.

Looking back on my terms in the U.S. Congress, in the California State Senate, and as Director of Finance for California, I am struck by how often arguments are won on the basis of how the numbers are presented. I am also struck by how great the temptation is for shading numerical data. Some of that temptation comes simply from the presenter’s lack of skill at presenting numbers. But some of that temptation also comes from the view—which, sadly, is all too often correct—that the audience will not speak up when they do not understand or are skeptical of the information put in front of them, and will, instead, be swayed simply by the amount, or exclamatory nature, of the data presented.

As Director of Finance for the State of California, I had to reconcile many different ways of presenting similar data from different departments of the state government. In that position, it was obvious that how clearly and coherently information is presented has a huge impact on how effective the presentation is. It was also obvious that a consistent approach to presenting the underlying numbers—in essence, creating a “common language”—is critical in a complex organization like the government of the State of California to making intelligent decisions, and to fostering a team approach to problem solving.

As a Dean in both public and private universities, I was introduced to new ways of presenting similar data, and my ability to comprehend the information was strongly affected by the way the data were presented.

In all of my roles, the ability to explain decisions to a larger audience—voters, students, faculty, trustees, legislature, press—demanded a need for simplicity, clarity, and transparent honesty. In the best-functioning organizations I have been associated with, the key players have had this ability when presenting statistics, financial data, and other numbers, and their audience has expected and demanded no less.

I’m delighted that a book like Painting with Numbers is available. I am confident that the lessons Randall Bolten sets forth in this book, a first of its kind, will have a hugely positive effect on the level of public debate, and thus, the quality of the choices we make in our public and private lives.

Tom Campbell


Everybody talks about the weather, but nobody does anything about it.

—Mark Twain

The concept for Painting with Numbers was one of those ideas that just emerged slowly over time. During my nearly 20 years as chief financial officer of high-technology companies in Silicon Valley, I saw small changes in documents containing numbers make a huge difference in how well they were understood. This happened over and over. In the case of financial documents, poorly understood reports could lead to mass confusion or even mistakes costing millions of dollars. But the right presentation of that same information could make an audience think aha! and motivate outstanding performance.

I also couldn’t help observing that the people generating these reports—extremely competent and intelligent people—often overlooked the nuances of these small changes and the effect they had on their audiences. When we reviewed the results afterward, though, they readily saw the connection. It became increasingly clear to me that presenting numerical information is a communication skill, and not some sort of black art practiced only by the “numbers guys.” I’ve stated that premise in hundreds of conversations since I began writing this book—with people ranging from accounting students and administrative assistants to CEOs and corporate directors—and the response has always been enthusiastic agreement. At the same time, though, very few of the people I spoke with had any sense of what that meant for how they should lay out numbers on a page, or a computer monitor, or a projection screen.

Think about it. A significant portion of our school years, especially in elementary and middle school, is devoted to training us to write and speak correctly, clearly, succinctly, and eloquently. We are taught grammar, vocabulary, diction, sentence structure, and paragraph organization. Even beyond our basic education, we are expected to continue to improve our grasp of the nuances of effective writing and speaking—about a particular subject, to a particular audience, with a particular purpose in mind. Bookstore shelves are loaded with books intended to sharpen those skills, with titles including The Elements of Style; Eats, Shoots & Leaves; The Careful Writer; The Business Letter Handbook; How to Write a Million-Dollar Memo; and Public Speaking: An Audience-Centered Approach.

Still, mathematics for the most part continues to be taught as a computation skill, not as a language for describing complex problems and situations. And although there are plenty of books about how to understand numbers, about the underlying mathematics, and even about how people try to fool you with numbers, there are no books focused solely on how to make yourself understood when you are presenting numbers.

Because the need to present numbers is critical in so many places in our society, I’m grateful and flattered that Tom Campbell agreed to write the Foreword for Painting with Numbers. His remarkable career has included public service as an elected legislator (U.S. Representative and State Senator) and executive branch appointee (California Budget Director when the state last reported a budget surplus); a Ph.D. economist, lawyer, and law professor; and a dean of both a business school (U.C. Berkeley, Haas School of Business) and a law school (Chapman University School of Law). Perhaps more important, he speaks and writes with precision, grace, humor, and a respect for his audience.

My goal with Painting with Numbers is to enable you to become as skilled at presenting numbers to an audience as the most articulate among us are at writing for and speaking to an audience. Whether you are an accounting, finance, or engineering professional who presents numbers as an essential part of your job; or you are a corporate law, litigation, human resources, marketing, investor relations, fundraising, or other professional who presents numbers less frequently (but when you do the stakes are high); or you are a government official or elected representative responsible for understanding and explaining important issues of fiscal management, taxation, healthcare, or other areas of public policy; or you are a senior manager or board member who must make complex decisions under time pressure based on the information presented to you—this book is for you. Enjoy!

Randall Bolten
Glenbrook, Nevada
February, 2012


First, my sincerest thanks go to Taylor Ray, editor extraordinaire, who understood what Painting with Numbers was about from the very beginning and stuck with the project through all its stops and starts. She was both immensely supportive and unfailingly candid, and worked tirelessly to make sure I honored my compact with the reader.

A few played special roles in the creation of this book. Joel Orr helped me get organized, explained the ins and outs of the publishing business, and offered wisdom that was rabbinical in the best sense of the word. John Cardozo demonstrated that a friend is someone who reads every word you’ve written even when he has more important things to do. And Tom Campbell not only agreed to apply its lessons to the electorate and lend his name to this book, he was the most punctilious, yet most insightful, witty, and supportive of readers—and this while running for statewide office in California.

Many people read parts of this book and made invaluable comments, including Stephen Few, Shomit Ghose, Dana Hendrickson, Karl May, Barbara McMurray, Larry Moseley, Ethan Thorman, Linda Wilson, and Donna Winslow. The finance team at Fortinet (Robert Lerner, Tim Emanuelson, Keith Andre, Jim Bray, Daneya Denson, Doug May, Scott Robinson, and Simran Singh) met with me frequently as a group to offer their from-the-trenches perspectives. To readers I somehow failed to mention, my apologies and my thanks.

My professional colleagues in the Silicon Valley chapter of Financial Executives International deserve special thanks as a group. Many made contributions to the content of this book, whether they knew it or not, and all indulged me patiently as I talked through my ideas.

I thank current math faculty at St. Albans School—Messrs. Kelley, Hansen, and Eagles—for their feedback on the book and for introducing its lessons to high school students. I also salute some of the teachers from my days as a student there, who made it clear that expressing yourself in your own language and style was not just tolerated, but a sacred obligation—especially Messrs. Means, Saltzman, Ruge, and McCune.

Many folks at Wiley deserve mention. Tim Burgard, Stacey Rivera, Andrew Wheeler, and Helen Cho were a real help to a first-time author and led me to intelligent choices. Natasha Andrews-Noel shepherded this book and a demanding author through a production process that was much more complex than I could possibly imagine. And my thanks go to Anne Ficklen for the introduction, and to Michael Rutkowski for just nailing the jacket design.

Some people less directly involved with the book itself deserve mention. A few comments made years ago still stick with me and helped spur me to this project: my appreciation goes to Jeff Walker, Woody Rea, Nort Rappaport, and Van Van Auken. I thank Claude and Sue and Leland, who offered excellent company and an indulgent ear, not to mention good food in the dead of winter. Family members Joshua, Susanna, Dede, and Jimmy provided feedback and cheerleading, and have assured me they will generate thousands of orders for this book.

One person was essential to this book in many, many small, and a few big ways. Well done, Petunia!

Even a book about expressing yourself with clarity, precision, and accuracy will inevitably have errors. For these I accept responsibility. I hope you will let me know about them. I also hope that you have gotten value from this book.


This Book is Not About Numbers—Honest!

The way you present says a lot about the way you think.

—Irwin Federman, venture capitalist

This book is not about numbers. This book is about presenting numbers, and doing it clearly, concisely, elegantly, and, most of all, effectively. This distinction between numbers and presenting numbers is critical, and to give you a glimpse of what’s to come, let me ask you some questions. When doing quantation,1 have you ever experienced the following:

If your answer is yes to any of these questions, then you’ve felt the agony and frustration of poor quantation. If you’ve been one of the “victims” in the audience,2 then you’ve known the sinking feeling of watching a presenter’s time and credibility painfully slip away.

Being “Literate”

Presenting numbers is the same as presenting any other information. If your audience can’t follow what you are presenting well enough to understand it, then you haven’t communicated. It doesn’t matter if your numbers are well-researched, organized with great effort, and correct. If no one understands what you’re trying to say, the problem is yours to fix.

Put another way, to do effective quantation you need to “be literate.” And I don’t just mean the more conventionally known definition of “being able to read.” Being literate means much more than that. Being literate is not merely about understanding words, but about comprehending any information presented to you—images, numbers, charts. It’s not just about comprehending information, but about how you present it. And it’s not just about what you know, but about people’s perceptions of what you know. In other words, being literate is about your knowledge, and your ability to present it so others perceive you as knowledgeable.

It’s What They Think That Matters

The previous point deserves some extra emphasis. An important part of being literate comes from other people’s perceptions of what you know. To help you understand how this works in real life, consider these questions:

Perception is important, and based on little things, people form global, and sometimes harsh, conclusions about you—about your intelligence, your literacy, your grasp of the subject matter, your professionalism, your ethical standards, and your respect for your audience (to name a few traits you probably value).

There’s one more thing about these little errors and embarrassments: the more important and influential people are, the more likely they are to form these snap judgments, and form them quickly. Not only are these people very busy, but they’re the kind of people who take pride in their speed and decisiveness. And the saddest part about the harshest conclusions is that people will rarely tell you. It’s like that little piece of spinach stuck between your teeth all evening at the annual company holiday party. No one mentioned it to you, but it sure was amusing conversation the next day in the hallways and the break room.3

So how do you avoid these negative perceptions and make sure that you are received as professionally as possible? If you’re communicating in words, you have lots of tools to guide you that you spent years learning: there’s vocabulary, grammar, diction, sentence structure, and paragraph organization. And if you use these tools well, your audience will consider you to be thoughtful and careful. But what about quantation? Are there rules and guidelines for presenting numbers? Of course there are, and that’s what this book is about.

What You Were Never Taught

The rules and practices that help you present numbers are similar to the rules and practices that make people effective writers and speakers. Unfortunately, very little of your education was probably devoted to developing these skills of quantation. This is especially unfortunate because typically when numbers are presented, significant decisions need to be made. These presentation skills are important even if you don’t present numbers often, because when you do have to present numbers, it’s likely that the stakes are going to be especially high.

Moreover, much of the value you reap from presenting numbers does not result from the time you spend compiling and generating the information, but rather from the time you take designing, organizing, and laying out the information you worked so hard to collect. The time that it takes to design, organize, and lay out your reports typically only takes 20% (or less) of your time. But then there’s that “80/20 Rule” that every MBA learns: 80% of the impact of any effort comes from only 20% of the time you spend on that effort. In a nutshell, this book is about spending that 20% of your time to your best advantage so you can become a better communicator of numbers, and therefore a better communicator in general.

- - - - - - - - - -
Let’s recap the key points so far:
1. When you present numbers, you are responsible for making sure your audience comprehends the information. Your audience will certainly hold you responsible.
2. If your audience fails to understand the information, your professional image and credibility will suffer. Moreover, you will waste time explaining what your information means instead of addressing your more important goals.
3. Small, innocent mistakes (in both language and numbers) have a disproportionately negative impact on how your skills, professionalism, and intellect are perceived by others.
4. The more important someone is, the more likely he or she is to reach these negative conclusions (about your presentation skills, professionalism, and/or intellect). And the more important someone is, the faster he or she will reach that conclusion.
5. All of the above points apply to presenting numbers as surely as they apply to any other communications; unfortunately, most of us haven’t been formally taught the skills to do effective quantation. That’s a shame, because quantation is a skill that can be taught and learned.
6. When a communication task involves quantation, doing it effectively is important both to the success of the task and to you personally:
  • When you are presenting numbers, the stakes are usually high.
  • If your job requires a lot of quantation (accounting, finance, or marketing analysis, for example) how you present numbers is a big part of your own professional image.
  • If your job doesn’t require much quantation, when you do have to present numbers, the stakes are probably very high.
7. The quantation task is the least time-consuming task in a project involving collecting, summarizing, organizing, and presenting numbers. If it’s done poorly, all of your other good work goes to waste. But if it’s done well, it becomes the most important and valuable task in the project.
8. Quantation is not about numbers; it’s about communicating to an audience (and you don’t have to be the resident “numbers guy” to do it well). Generating clear, effective quantation is what this book is about.
I chose “The way you present says a lot about the way you think” as the epigraph for this Introduction because it relates to all of the reasons listed above. Perhaps Mr. Federman was specifically referring to standing up and giving an oral presentation, but I believe it is apt for any way you present information. When you present numbers, you expose every aspect of your thought process (or lack thereof) to your audience.
Please note: Nothing in this book is intended to be used as a cookbook. My goal is to give you the tools to enable you to design reports yourself that meet your quantation needs. Next is one example, of a printout I found taped to a table in the lobby of a county clerk’s office. By the end of this book, you’ll be able to identify everything that’s wrong with this document, and you’ll know how to improve it.
The Ultimate Goal
As any good artist or teacher will tell you, you don’t create a great painting because you went to the best art schools, used all the available colors, or knew the latest brushstroke techniques. The test of a great painting lies more in the answers to questions like: Does the painting tell a story? Is the viewer led to care about that story? Has the artist revealed the essential nature of the subject, regardless of how big or small the canvas is? My goal in Painting with Numbers is to give you all you need to know to communicate financials and other numbers so that you can tell your story in a way that is clear, comprehensible, concise, elegant, and, most of all, effective.
- - - - - - - - - -

Some Notes about Reading This Book

Painting with Numbers covers what you need to know to do quantation effectively. It’s divided into four sections: The Rules, The Tools, Real Mastery, and Wrap-Up.

Part I: The Rules discusses the rules and practices that can help you “be literate” when you’re doing quantation—much the same way understanding grammar and using it effectively help you communicate and “be literate” when you write or speak. It’s critically important for you to understand why these rules and practices work, so I’ll discuss that as well. The topics covered in this section are:

Part II: The Tools offers a practical, tactical discussion about how to use the principal software tools to maximum advantage in your quantation.

Part III: Real Mastery addresses the importance of subject-matter expertise when delivering quantation, examines practical applications of quantation, and provides examples of widely used reports, with particular emphasis on applying the rules and techniques from the chapters in Part I.

Part IV: Wrap-Up is exactly that. Chapter 15 includes my thoughts on ethics, honesty, character, and other cosmic issues, and how all that relates to quantation. Chapter 16 provides a summary of all that we covered, and includes a discussion of how good quantation begins. In the Appendix, we revisit all of the Deadly Sins (defined momentarily) in one place, and from some unusual perspectives.

Important Elements to Aid Your Memory. Throughout Painting with Numbers you will find structural elements intended to drive home the main points of the book. These elements can be collected in quick lists for future reference. The principal elements are “The Deadly Sins of Presenting Numbers” and “Strong Advice,” both of which appear sprinkled throughout these pages. There are also chapter-specific elements, including the “Instant Payoff Tips” and the “Long-Term Payoff Tips” for efficient and effective spreadsheet delivery, and the “Characteristics of a Well-Designed Natural P&L.”

The Deadly Sins of Presenting Numbers is my metaphor to drive home the notion that seemingly small errors can say more than you ever want (or deserve) about your thought process. The Deadly Sins are quantation errors that you should avoid at all costs, because they are:

As an antidote to the Deadly Sins, from time to time I introduce suggestions labeled as Strong Advice. I make numerous suggestions in these pages, and most of them are just that—suggestions—but the “Strong Advice” items are counsels that you should always follow. When you do, they will make your quantation powerful indeed.

The chapter-specific elements are groups of suggestions that you will find useful in highly tactical situations. The “Instant Payoff Tips” and the “Long-Term Payoff Tips” will be of use in making you a faster spreadsheet user. In fact, you might find them most useful if you’re not a frequent or regular spreadsheet user and haven’t had a chance to build up your habits and your memory of the tricks of the trade. The “Characteristics of a Well-Designed Natural P&L” in Chapter 11 is a list of characteristics that will be useful when you have to design a management P&L, but also when you have to manage your organization from reports provided to you.

The Exhibits: No Previous Accounting Experience or Graphics Arts Talent Needed. The majority of the exhibits and corresponding discussions relate to a variety of business situations, but I have designed them so that the discussion will be meaningful even if you have had only a passing exposure to financial statements and no technical accounting experience at all. Moreover, the purpose of the exhibits is to illustrate presentation techniques and not the underlying accounting. So if, when it comes to numbers, you think of yourself as a “poet,” take heart—this book is for you, too. There may be a couple of chapters late in the book that you might choose not to focus on, but that would be because they lack relevance to you, not because they’re over your head.

Just as I believe that financial and accounting experience are not prerequisites to reading and understanding Painting with Numbers, neither is any skill in graphics arts. Every single exhibit in this book was created by me, a real non-artist, using off-the-shelf Microsoft Excel 2010 with an ordinary font (Arial 10 points), and then pasted into the pages of this book. In other words, these exhibits look exactly like reports that you might generate yourself in Excel. I’ve tried to make sure that the print in all of the Excel exhibits was sized equally; because of the space limitations of a book page, that meant a somewhat smaller print size than you would see on a paper printout, but the exhibits are still easily readable. For the few exhibits that needed further size reduction, my apologies, but please understand that the main point for those exhibits was about layout, and the readability of the numbers themselves was of less concern. The spreadsheet design practices used here are nothing more than habits I’ve developed in the nearly 30 years since I was first exposed to spreadsheets. Producing good-looking, clear, and effective quantation doesn’t require talent, but it does require attention to detail, a passion for delivering a high-quality product, and—something that gets more than a passing mention in this book—building habits and setting standards for yourself that enable you to produce beautiful information every time you sit down to design a report.

The Footnotes—Sometimes Informative, Sometimes Entertaining, Not Necessarily Essential. Throughout this book I have added technical and other details that you might find useful. They offer historical or theoretical context for points made in the text, or they are my own opinions or other relevant personal observations. While I hope you will find them interesting and valuable, I present them as footnotes because they are not essential to the narrative flow of the text, and you will not miss any of the fundamental essence of Painting with Numbers by ignoring them.


1. quantation (kwn-t’-shn) n. [English, c. 2008, from QUANTitative + communic-ATION]. The act of presenting numbers, such as financial results, electronically or in written form for the purpose of informing an audience. (Note: Quantation is the word I coined to describe the subject of this book, because no single word for it exists. It is not a word you will find in the dictionary, as of this writing.)

2. By “audience,” we don’t just mean a live audience at an oral presentation. Your audience could be reading hardcopy printouts, receiving e-mail messages, or sitting right next to you.

3. With these types of reaction in mind, if you don’t know whether its or it’s is appropriate in any given situation, do yourself a favor: Find out. Right now. It’s a mistake that rarely makes a sentence hard to understand, but even so it might be the English grammar mistake most likely to cause you to be perceived as stupid and illiterate. For a similar perspective on this topic from the perspective of someone who cares deeply about the quality of written English, I highly recommend Eats, Shoots & Leave: The Zero Tolerance Approach to Punctuation, by Lynne Truss. A close second to the its/it’s error are the your/you’re exacta and the there/their/they’re trifecta. Moreover, if you’re relying on the built-in error-checking features in today’s word processing software to protect you from boneheaded mistakes, guess again—all of the mistakes I’ve cited are very unlikely to be caught by built-in software.