July 10, 2012

Epidemiologists call them “absolute risks”; you and I might call them the real numbers

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In a recent Huffington Post blog post, Dartmouth’s Gil Welch addressed an old, pet theme of ours, “The Problem is Relative.” Excerpt:

Numerous studies have shown that the general public has exaggerated perceptions of the health risks they face — as well as exaggerated expectations of the benefit of medical care.

Is it because they’re stupid? No. Instead, the problem relates to how various sources of health information — researchers, doctors, reporters, web designers, advertisers, etc. — frequently frame their messages: using relative change.

“Forty percent higher” and “50 percent lower” are statements of relative change. While they are easy to understand, they are also incomplete. Relative change can dramatically exaggerate the underlying effect. It’s a great way to scare people.

For example, research earlier this year found that women with migraines had a 40 percent higher chance of developing multiple sclerosis. That sounds scary.

But the researchers were careful to add some important context: Multiple sclerosis is a rare disease. In fact, for women with migraines, the chance of developing multiple sclerosis over 15 years was considerably less than 1 in 100 — only 0.47 percent. To be sure, that is about 40% higher than the analogous risk for women without migraines — 0.32 percent — but it’s a lot less scary. More importantly, it’s a much more complete piece of information.

What makes it more complete is the context of two additional numbers: the risk of developing multiple sclerosis in women with and without migraines. Epidemiologists call these “absolute risks.” You and I might call them the real numbers.

Relative change also exaggerate effects in the other direction. It’s a great way to make people believe there has been a real medical breakthrough.

When we grade stories on HealthNewsReview.org, we usually give them an unsatisfactory score for the “Quantifying Benefits” or “Quantifying Harms” criteria if they use only relative risk figures.

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