One in nine people will misunderstand statistics

Have you seen those ads for the Weekend to End Breast Cancer; the ones with the tagline “Breast cancer is hard; walking is easy”?

As a prostate cancer researcher and advocate for increased funding for prostate cancer research, I really have to hand it to the breast cancer fundraising community; they have done an absolutely outstanding job at raising awareness and especially in raising money for breast cancer research. We in the prostate cancer community (which has about the same incidence and mortality as breast cancer) could learn a thing or two about how to take a disease that was not discussed and elevate awareness and funding for it to unimaginable heights. In 2005, breast cancer received about 15% of all cancer research funding in Canada and accounted for approximately 12% of all new cancers and 7% of all cancer deaths in 2003. By contrast, colorectal, lung, and prostate cancer, which each accounted for approximately the same number of new cases of cancer (and in the case of colorectal and lung cancers, far more deaths than breast cancer), each received 3-6 times less funding than breast cancer. There is no doubt that the massive public awareness campaign for breast cancer and breast cancer research fundraising have played an enormous role in this. Once again, I say this not to disparage the breast cancer advocacy community, but rather to sound the alarm that a similar initiative is needed in prostate and colorectal cancers, both of which have a slight public image problem due primarily to the anatomical location of the affected site.

So kudos to the breast cancer community for getting it right. And shame on the breast cancer community for getting one thing completely and utterly wrong.

If you’ve heard the commercial, you may remember one line in particular:

“One in nine women will get breast cancer.”

This is a shocking statement; one clearly designed to elicit a response. Look around you in the room you’re in; this ad suggests that if there are nine women in the room, at least one of you will develop breast cancer.

The problem, of course, is that this is completely misleading.

In 2007-2008, there were 364,085 births in Canada. If we assume 50% of these births were of females, that makes 182,040 new Canadian girls. There were an estimated 22,600 new cases of breast cancer diagnosed in Canada in 2008, which means about 12.4 cases per 100 girls born, or about 1 case for every 8 female births. Everything seems right so far (even slightly scarier at 1/8 instead of 1/9). There’s a problem, though.

The incidence of breast cancer, as with most cancer, is not evenly distributed. For instance, about 42% of all new cancer cases and about 60% of all cancer deaths will occur in people over 70 (Canadian Cancer Society, 2008 Statistics).

The most telling way of assessing one’s actual risk of developing cancer is to determine the probability of getting cancer in the next ten years, given one’s current age. Luckily, we have a very committed group of people at the Canadian Cancer Society (some of whom are former colleagues of mine) who dedicate their time to do these types of calculations.

On page 52 of the 2008 Canadian Cancer Society Statistics book (found here), you can see just these sorts of calculations.

A woman’s lifetime risk of developing breast cancer is, indeed, one in 9.1 (11%). But the risk of a 31 year-old woman developing breast cancer in the next 10 years is only 0.4% (or less than 1 in 100). As the woman ages, this risk obviously rises (though at a slower rate than has been the case in previous years, thanks in part to the reduction in Hormone Replacement Therapy use), until she hits age 70. At age 70, she has a 3.1% chance of developing breast cancer, or about one in 32. Yes, all you breast cancer-free 70 year-olds out there; your risk of developing breast cancer in the next ten years is only one in 32. And the 80 year-olds? Their risk is even lower; one in 40. How can this be? Simple; most 80 year-old women are developing other diseases and dying of other causes.

The general public cannot be expected to perform this type of exercise every time a commercial comes on TV. However, The Campbell Family Foundation for Breast Cancer Research, who hold the Weekend to End Breast Cancer and are the recipient of the funds, should know better.

So what is the correct statement?

“One in nine women, who live to their full life expectancy, will develop breast cancer.”

Not quite as snappy, but far more accurate, and not nearly as misleading.


5 Responses to One in nine people will misunderstand statistics

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