Look at the circles in the middle of these two illustrations. The one on the left is bigger, isn’t it?
No. The circles in the middle are exactly the same size. The one on the left looks bigger, but this is just an optical illusion. The small circles in the lefthand picture make the middle circle look relatively big, while the big circles in the righthand picture make the middle circle look relatively small. The point is that people do not interpret information in isolation.
Even when news stories, ads, or public service announcements do a good job of providing risk statistics (that is, including the number of people who experience the outcome, the number who could experience it, and the time frame), you still need more information to make the numbers meaningful. The numbers need to be put in perspective. One kind of perspective, described in the previous chapter, lets you compare your chance of getting a disease with your chance of dying from it. This sort of perspective helps you appreciate how dangerous the disease is—and, for many people, knowing their chance of dying from a disease is really the most important information.
For much of this book, we focus on the chance of dying. In this chapter, we provide ways to get perspective on this chance. We’ll start by taking age and sex into account to help you see how a risk applies to you as an individual. Finally, we’ll add another kind of perspective: how one disease compares to others.
In this chapter and the next, we’ll introduce you to the “risk charts” that provide this perspective. These charts show the chance that people of different ages will die from various causes. We created these charts using data from the federal government’s vital statistics records—the best available data.
About 5 years ago, we published the charts and our method for calculating them in the Journal of the National Cancer Institute.1 Since then, we have updated the charts using the most recent data available (which is from 2004) and refined our calculations. The new charts have just been published in a recent issue of the Journal of the National Cancer Institute.2
We’ll start with a small excerpt of the chart showing the chance that women of different ages will die from colon cancer in the next 10 years. To use the chart, begin with the first column and find the age you’re interested in. Then look in the next column to see the number of colon cancer deaths that will occur among 1,000 women that age in the next 10 years.
Table
The numbers in the chart tell you how many out of 1,000 women of various ages will die in the next 10 years from colon cancer.
To be sure you understand how to use the chart, please answer the following question:
If you’ve read the chart correctly, you’ll see that the answer is c, 1 out of 1,000. We’ll be using this kind of chart a lot, so it’s important that you understand how to read it.
As you can see, this risk increases with age. For example, by age 75, a woman’s chance of dying from colon cancer in the next 10 years is 10 out of 1,000.
A Broader Perspective
A 50-year-old woman has a 1 out of 1,000 chance of dying from colon cancer in the next 10 years. Is that a large or small chance? To decide, it helps to compare this risk with other risks. Comparing the risk of dying from different diseases helps you appreciate which are the biggest threats you face (the ones you might want to do something about) and which threats are less worrisome.
The next chart compares a woman’s risk of dying from colon cancer to her risk of dying from other causes. For simplicity, we show only the information for 50-year-old women.
Table
The numbers in the chart tell you how many out of 1,000 fifty-year-old women will die in the next 10 years from . . .
Let’s make sure you understand how to use the chart:
The correct answer is b, heart attack. The chart shows that a 50-year-old woman’s chance of dying from a heart attack in the next 10 years is 4 out of 1,000, while her chance of dying from colon cancer during that time is 1 out of 1,000. Her chance of dying from colon cancer is about the same as her chance of dying from stroke or ovarian cancer but is higher than her chance of dying from cervical cancer, pneumonia, or AIDS.
The correct answer is d, more than 14 out of 1,000. Sorry—this may have seemed like another trick question. Of course, the chance of dying from all causes combined is greater than the chance of dying from colon cancer. To get the correct answer, however, you had to realize that there are many other causes of death besides the ones included in the charts (for example, homicide, suicide, other cancers—such as stomach cancer, bone cancer, or melanoma—kidney failure, infections, and so on). So the number had to be greater than 14, which is the sum of the numbers listed in the chart. The actual answer is 37 out of 1,000.
Knowing the chance of dying from all causes combined adds another important perspective: it allows you to see how each individual cause of death contributes to this total. Because there are so many causes of death, you can’t simply add up the numbers in the row. So we now add a final column on the righthand side of the chart: the chance of dying from all causes combined.
Table
The numbers in the chart tell you how many out of 1,000 fifty-year-old women will die in the next 10 years from . . .
To decide how you really feel about a risk message, remember to give yourself an opportunity to react to both the “glass half-empty” and “glass half-full” versions. In the previous chapter, we mentioned framing, the idea that alternate ways of presenting the same information can elicit very different feelings. We suggested that when you hear a risk reported as the chance of dying, you might try reframing it as the chance of not dying. We have done just that in the following charts. Each chart indicates what is likely to happen to 1,000 fifty-year-old women over the next 10 years.
A
The chance of dying in the next 10 years.
B
The chance of not dying in the next 10 years.
Remember, to understand risk, you need to know the outcome being considered, the size of the risk, and the time frame. And it’s important to be aware of how the risk is presented (the framing). But in addition you need perspective: a way to compare this risk with others. Comparing makes it possible for you to decide how big or important a risk really is. Perspective really matters. But don’t just take our word for it:
DILBERT © Scott Adams / Dist. by United Feature Syndicate, Inc.
Publication Details
Copyright
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.
Publisher
University of California Press, Berkeley (CA)
NLM Citation
Woloshin S, Schwartz LM, Welch HG. Know Your Chances: Understanding Health Statistics. Berkeley (CA): University of California Press; 2008. Chapter 2, Putting Risk in Perspective.