American women fear breast cancer more than heart disease, according to most studies, even though heart disease is responsible for 10 times as many female deaths every year -- and heart disease deaths exceed breast cancer deaths in every decade of a woman's life.
Of women who are diagnosed early with breast cancer, more than 90% will survive, and most will not need disfiguring mastectomies or even chemotherapy. But the media understand how deeply women fear breast cancer, and the result is that every study that seems to find a link between some new risk factor and the disease makes headlines everywhere, captures public attention and stimulates the blogosphere into overdrive.
Grapefruit is the most recent culprit. According to a study in the Journal of the American Medical Assn., eating a quarter of a grapefruit a day increases the risk of breast cancer by 30%. For many women, grapefruit immediately was toast.
To assess these studies for their real-life implications, let alone for making decisions about our own behavior, the public needs to understand the difference between absolute risk and relative risk. If we tell you that the relative risk of breast cancer is increased by 300% in women who eat a bagel every morning -- Relax! It's not! -- that sounds alarming, but it is not informative. You would need to know the absolute numbers of bagel-eating breast cancer patients. If the number shifted from one in 1,000 women to three in 1,000 women, that is a 300% increase, but it's meaningless. If the risk had jumped from 100 women to 300, we might reasonably be concerned.
In the large epidemiological studies that generally include tens of thousands of people, it is very easy to find a small relationship that may be considered "significant" by statistical convention but that, in practical terms, means little or nothing. For example, in July 2002, the Women's Health Initiative reported a 26% increase in breast cancer risk for women on hormone replacement therapy, which sounded worrisome. Even if that number were statistically significant -- and it was not, by the way -- this is what it translates into: The risk of breast cancer would increase within the studied population from five in 100 women to six in 100 women.
