Saturday, August 15, 2009

How to Tell Well-Grounded Fears from Groundless Ones

Thanks to one of my new gurus, Sandy Szwarc of Junkfood Science:

Just imagine how many popular fears and health agendas would disintegrate in an instant if the public realized that relative risks less than 10 — that’s 10-fold or 900% as high — with p-values >0.01 are often not real, tenable and are generally explained by confounding factors.

We’re so used to hearing inconsequential relative risks reported as NEWS that we’ve come to believe they are real and worth us acting upon. Instead, when we hear reports of these small relative risks, if we sat back and waited for the science to work itself out, we’d be a lot less likely to get caught up in the claim of the day and be taken advantage of.

This new guideline may sound like an extreme idea, but perhaps not when we look at the relative risks derived in epidemiology that have later proven out in clinical studies, versus all of those that haven’t.

A few examples may help put risks into perspective. “Studies of heavy smoking and lung cancer report a relative risk of about 20; those of aspirin and Reye's syndrome in children report a relative risk of 35,” said Steve Milloy of Junkscience and author of Science Without Sense. The FDA Center for Food Safety and Applied Nutrition reports relative risks for listeriosis associated with raw seafood of 17 among most adults, rising to 20 in elderly; and 15 with unpasteurized milk. The relative risks for Kaposi sarcoma associated with HIV infection is 192 (95% confidence interval); and the relative risks for non-Hodgkin lymphoma with HIV have been reported as high as 76.4. Among carriers of the BRCA1 mutations, the relative risks of breast cancer have been estimated to be 21.6 in women under 40 years of age, 9.6 in women 40-49 years of age and 7.6 in older women.

As Dr. Lloyd wrote, in commenting how cohort studies are often wrong, yet acted upon in isolation:
Glasziou et al suggested that a combined rates ratio of at least 10 and a P value of <0.01 should be used to distinguish between a true effect and background population “noise." Few of our current favourite targets - mild [sic] obesity, salt intake or passive smoking — would pass this test. The findings of cohort studies should start rather than close the debate. Experts are too hasty to present a hypothesis as a proven fact, and the medical profession is too willing to accept such findings uncritically.

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