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Woloshin S, Schwartz LM, Welch HG. Know Your Chances: Understanding Health Statistics. Berkeley (CA): University of California Press; 2008.

Cover of Know Your Chances

Know Your Chances: Understanding Health Statistics.

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What This Book is About

Every day we are faced with news stories, ads, and public service announcements that describe health threats and suggest ways we can protect ourselves. It’s impossible to watch television, open a magazine, read a newspaper, or go online without being bombarded by messages about the dangers we face.

Many of the messages are intended to be scary, warning us that we are surrounded by danger and hinting that everything we do or neglect to do brings us one step closer to cancer, heart disease, and death. Other messages are intended to be full of hope, reassuring us that technological miracles and breakthrough drugs can save us all. And many messages do both: they use fear to make us feel vulnerable and then provide some hope by telling us what we can do (or buy) to lower our risk. In addition, as you may suspect, a great many of these messages are wildly exaggerated: many of the risks we hear about are really not so big, and the benefits of many of the miraculous breakthroughs are often pretty small.

As a result, we are often left misinformed and confused. But it doesn’t have to be that way.

The goal of this book is to help you better understand health information by teaching you about the numbers behind the messages—the medical statistics on which the claims are based. The book will also familiarize you with risk charts, which are designed to help you put your health concerns in perspective. By learning to understand the numbers and knowing what questions to ask, you’ll be able to see through the hype and find the credible information—if any—that remains.

Don’t worry: this is not a math book (only a few simple calculations are required). Instead, this is a book that will teach you what numbers to look for in health messages and how to tell when the medical statistics don’t support the message. This book will help you develop the basic skills you need to become a better consumer of health messages, and these skills will foster better communication between you and your doctor.

Confusing Health Messages

Unfortunately, it can be hard to make sense of health messages. It can be difficult to figure out just how big the threats are or how well the drugs, tests, or behaviors highlighted in the messages actually work.

Why is it so hard? First, there are problems with the messages: many are incomplete, misleading, or overstated. It’s easy to understand why. The message writers may not know what they’re doing—or, more likely, they may know exactly what they’re doing. Without doubt, the media, medical journals, pharmaceutical companies, researchers, research funders, and academic institutions all have an interest in being associated with work that is perceived to be big, new, and important. That is a recipe for exaggeration.

Second, there are problems with the audiences who receive the messages. Most people haven’t been taught how to “read” health messages critically—in other words, they don’t know which numbers to look for, how to recognize when key data are missing, how to put messages in perspective, or how to think about the evidence behind the messages and assess what counts as credible evidence.

This book will teach you how to look at health messages critically and how to understand the statistics behind them. We want to help you develop a healthy skepticism that, as illustrated here, will let you push back against unfounded, exaggerated claims. We are not saying that there’s never anything to fear or that there’s no reason for hope. We are saying that it takes some effort and skill to see through exaggeration and to recognize real dangers and benefits.

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A Quick Word on Words

Before we go further and start looking at health messages, we want to ensure that we’re being clear about some basic language.

Risk: To most people, the word risk means “danger”: for example, “lion taming is a high-risk hobby.” But risk is also used to mean the chance that something will happen to you: for example, “in this group of patients, the risk of heart attack is about 10 percent.” In this book, we use risk as a synonym for chance. Usually, we’ll be talking about the chance of something bad happening (like having a heart attack). Strictly speaking, risk can refer to the chance of anything—good or bad—happening. This leads us to the next key word, outcome.

Outcome: The outcome is the “something” that may happen—maybe death or maybe a medical event such as a heart attack or a cancer diagnosis. Thus, risk is the chance that you will experience some outcome. For example, a man may be interested to know his risk of prostate cancer. That is, he wants to know the chance of something occurring. The “something” here—the outcome—is getting prostate cancer.

Statistics: The last word we want to introduce is one that may strike fear into the hearts of many readers: statistics. Statistics are just numbers that summarize information. They are based on observations of large groups of people and are useful in predicting what is likely to happen in the future. The statistics we focus on summarize the chance that an outcome will happen. These risk statistics are simply fractions. The numerator (the top number in the fraction) is the number of people who actually experience the outcome. The denominator (the bottom number) is the number of people who could potentially experience it. For example, if 10 lion tamers get bitten among the 100 lion tamers that we observe for a year, the fraction looks like this: 10/100. That is, a lion tamer’s risk of being bitten is 10 out of 100, or 10 percent per year. The risk statistic here summarizes the chance that a lion tamer will experience the unpleasant outcome of getting chomped on by a lion.

Statistics can be complex and tricky—but they don’t have to be. And as this book will show you, good statistics usually aren’t.

An Overview of What Comes Next

This book is intended for anyone who encounters health messages—in other words, for everyone. We don’t aim to turn you into doctors or statisticians; rather, the goal is to help you make sense of the messages. Because many people are intimidated by statistics, we worked hard to make the book non-threatening and engaging, by including plenty of illustrations, walking you through examples, and offering the more technical explanations in separate sections.

The book is divided into four basic parts, with a final section that provides extra help.

Part One: What Is My Risk?

The first chapter focuses on how to analyze messages about risk—specifically, how to find the fundamental information needed to give the message meaning, and how to do one or two simple, relevant calculations. You’ll learn to ask the following questions: “Risk of what? How big is the risk? Does the risk information reasonably apply to me?” We’ll also look at how messages are framed: subtle changes in how risk messages are presented can make a big difference in how people interpret—and act on—identical information.

Chapter 2 highlights a key issue: people are typically given risk information in isolation, with no idea of how a particular risk compares to other important health risks. This chapter stresses the importance of perspective in interpreting risk messages. To illustrate perspective, we introduce risk charts, simple tables that show a person’s chance of dying over a 10-year period from various causes (for instance, heart attack, stroke, pneumonia, flu, accidents, or cancer) and from all causes combined.

Chapter 3 explains how to use the risk charts and reinforces the importance of perspective in interpreting risk information. Specifically, we focus on how basic characteristics such as age, gender, and smoking greatly influence many risks. We’ll present two risk charts—one for men and one for women—that indicate the chance of death for people who have never smoked and people who currently smoke. By comparing risks of death from various causes, and all causes combined, you can get some perspective on the relative importance of these risks at different ages. Also, by comparing the current smoker and nonsmoker risks, you’ll understand why all doctors agree that not smoking is the single most important thing you can do to prolong life.

Part Two: Can I Reduce My Risk?

Just as chapter 1 teaches you how to understand risk messages, chapter 4 teaches you how to understand messages about risk reduction. You’ll learn to ask these key questions: “What outcome is being reduced? And how big is the reduction?”

Chapter 4 also builds on the idea of figuring out just how well the information in a health message applies to you. Like messages about risk, messages about risk reduction are most relevant if they are based on studies of people like you. This chapter emphasizes a key point: the size of the risk reduction you can expect from an intervention depends on your starting risk—in other words, the higher your risk to begin with, the more you stand to benefit from an intervention.

Understanding the benefit means understanding the importance of different outcomes—the way in which benefit is measured. Chapter 5 introduces you to the pyramid of benefit, which can help you assess the value of interventions, ranging from the least important interventions, which improve only things that you don’t directly experience (such as blood test results) to interventions that change how you feel or how long you live.

Part Three: Does Risk Reduction Have Downsides?

Treatments always have downsides: cost, inconvenience, bothersome side effects, or, rarely, major life-threatening consequences. Chapter 6 teaches you how to think about side effects in order to decide whether the health interventions you hear about are worth considering.

Chapter 7 provides practical advice on how to put the lessons of the previous chapters into action: how to weigh the benefits and downsides of treatments. This means determining how large the risk reduction can be and what you have to do to achieve it. Treatments that have a lot of downsides are worth doing only if they have a lot of benefit.

Part Four: Developing a Healthy Skepticism

As we said before, the health messages you encounter every day in the media are often misleading and overstated. Chapter 8 reviews the strategies that the authors of these messages sometimes use to exaggerate the importance of risk (or risk reduction) and provides some guidance on how to be a healthy skeptic. We also take a look at some of the most commonly misused (and exaggerated) statistics around: survival statistics. These numbers tell you what percentage of people are still alive at a specified time after a diagnosis (typically, you hear about 5-year or 10-year survival). Although survival statistics can be very important when they are used correctly, these seemingly simple numbers are easily misused and can cause lots of confusion. By the end of chapter 8, you’ll know how these statistics work and how to tell when they are being abused.

Chapter 9 reminds you that getting the numbers is not the whole story. You must still decide whether or not to believe them. We offer some guidance about how to judge the believability of research findings and a set of cautions to keep in mind when you hear about different kinds of research (for instance, observational studies or randomized trials). The chapter ends with a caution about interpreting preliminary findings, such as research presented at meetings of medical or scientific associations. Because such preliminary findings are not the final results, the conclusions of the study may change (a lot) over time as the study matures.

In chapter 10, we urge you to think about who is behind the numbers you hear in health messages. Some of the sources of the information you encounter have important interests besides your health. Unfortunately, these other interests may tempt the people behind the numbers to distort or to spin the information to make their message look more compelling—for example, by maximizing the apparent benefit or by minimizing the potential harm of interventions. Many conflicts of interest are financial, but some are non-financial: scientists and physicians may benefit professionally by being associated with work that appears to be groundbreaking, new, or important. When you evaluate the credibility of the scientists and the research behind a health message, it’s worth asking whether any of these conflicts exist.

Extra Help

Finally, the Extra Help section provides tools that you can use to make sense of health messages in the future. It includes a quick summary of the entire book, a glossary of key terms, a number converter that helps you rewrite numbers into an easy-to-understand format, and the full version of the risk charts discussed earlier in the book. We have also provided a list of credible sources for health statistics and a section of notes with references for some of the material we present.

How to Use This Book

Much of the information in this book may be new to you. That’s okay. It’s new to lots of people—including many doctors. But we’re confident that almost everyone can learn to make better sense of the health statistics that we see in real life. To help you along, we have included the following features:

Quizzes are meant for everybody. Do them before you look at the answers. Quizzes will help you know how well you’ve understood the material—and help build your confidence. We urge you to do them and not to proceed until you understand the right answer, since what comes next often builds on what came before.

Learn More boxes are meant for readers who want to investigate topics a little more deeply. You can safely skip these boxes and still get the big picture, but we think you’ll learn more if you take the extra time to read them.

We have tried to make this book short and simple. But there is still a lot here. You’ll do best if you take time to digest and reflect on what you learn. Try reading the book in a few sittings, and take some intermissions. Good luck!

Copyright © 2008, The Regents of the University of California.

Know Your Chances: Understanding Health Statistics is hereby licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported license, which permits copying, distribution, and transmission of the work, provided the original work is properly cited, not used for commercial purposes, nor is altered or transformed.

Bookshelf ID: NBK126164


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