Health check: Innumeracy breeds fear

  • @jeremylaurance
IF YOU held a lunch party and 24 people turned up what would you reckon the chances were that two of them shared the same birthday? A hundred to one? No. Fifty to one? Nope. The answer, according to my Sixties edition of Ripley's Believe It or Not, is 50-50. In other words, at roughly half of all events attended by 24 people, two of those present will celebrate the anniversary of their birth on the same day.

So what? Well, to most people, myself included, this comes as a surprise, and it illustrates how poor we are at making judgements involving statistics. That is why bookmakers enjoy a good living. It also explains how figures can be used to frighten people.

The best example is breast cancer, a disease that women, understandably, fear more than any other. I say "understandably" because the figures bandied about - one in nine women will get the disease and one in 12 will die of it - make it sound frighteningly common. But it is important, both for women's health and for what we as a nation decide to invest in it, that we have a clear perspective on the risks.

An excellent review in a recent New England Journal of Medicine spells out just what the one in nine figure means. For any group of nine women, breast cancer will develop in one of them at some time in her life, but not in the remaining eight.

Most women tend to interpret this risk as a short-term probability and grossly overestimate their chances of falling victim over any 10-year period. A women entering her thirties has a one in 250 chance of developing the disease in the next decade. In her forties it is one in 77. Although the risk of contracting breast cancer increases with age, the risk in any decade never exceeds one in 34.

Breast cancer is only one risk among many, so it needs to be set in the context of other risks. At any age, the cause of death among women is always likely to be something other than breast cancer. In fact, although the incidence of the disease increases with age, the proportion of deaths caused by breast cancer never exceeds 20 per cent.

To illustrate this, the authors used a life table based on figures from the Ontario Cancer Registry. Of 1,000 women born in the same year, an average of 17 will die by the age of 40, none of them from breast cancer. In their forties and fifties, 54 will die - nine from breast cancer and nine from heart disease and stroke. In their sixties and seventies 305 women will die, 105 from heart disease and 18 from breast cancer.

By the age of 85, 203 women will have died of heart disease and 33 of breast cancer. However, 99 will have developed the disease, approaching the one in nine statistic.

This demonstrates how heart disease and stroke exact a far greater toll than breast cancer. Yet a survey by the National Council on Ageing in the US found that just 9 per cent of women said the condition they feared most was heart disease compared with 61 per cent who said cancer (mainly breast cancer), and there is no reason to suppose that British perceptions would be different.

This distorted view may have important consequences for women's health. Aside from the obvious point that they can, if they are so minded, do something to reduce their risk of heart disease, by changing their lifestyle, women who might benefit from HRT may choose not to take it because of their fear of breast cancer.

Breast awareness campaigns typically use images of young women and are targeted at 20-40-year-old readers of women's magazines. But two facts are worth stressing. Breast cancer is predominantly a disease of older women; and for most of them it is not lethal.