One of the questions most often asked in the medical literature is “what is the risk of x?” It’s a pretty important question. I’d like to be able to tell my patient with high blood pressure what their risk of heart attack is, both with and without treatment. And risk is a sexy topic—the press loves it. Whether it’s cell phones and the “risk” of brain cancer, or vaccines and the “risk” of autism, risk makes for cool headlines. Take this one for example:
Migraines cut breast cancer risk 30 percent: study
What does this mean? Should I tell my wife to go out and find some migraines? I have a feeling one or more of my colleagues will give a more detailed critique of this study, but I’d like to talk to you a bit about what we mean by “risk”.
Risk, in the most basic sense, is a causal association. If, for example, I find that members of the “Thunderstorm-lovers Golf Association” have a higher incidence of being struck by lightning than golfers who don’t belong to this odd club, I may have stumbled upon a measurable risk. There is both a measurable association, and a plausible reason to causally link the associated variables. If I find that members of the National Association of Philatelists have a higher incidence of heart disease than other folks, I may or may not have stumbled on a risk. Is there a reason that philatelists should have more heart disease? Is it a coincidence? Is it worth investigating further? Is there a confounding variable, e.g. are philatelists in general older, and did I fail to control for this?
Then there is the question of the degree of risk. How strong is the risk observed?