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Maybe I'm missing the point of this entire thread and don't truly grasp the argument for EBM vs. correlation, but can someone explain why a statistically significant correlation for this kind of efficacy is not evidence in and of itself?

http://www.sciencedirect.com/science/article/pii/S0001457500...

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Those aren't prospective randomized controlled trials. They're retrospective observational studies. They are evidence of efficacy, no doubt, but not particularly convincing evidence for all the normal reasons observational studies are often unconvincing, but maybe even moreso in light of some other contradictory results in the literature as well as the general concept of risk compensation (i.e., it's not like a parachute where it's very clearly obvious that it works).

i believe (from memory) that the argument is that wearing helmets deters people from cycling which has a cost in terms of deaths from conditions related to lack of exercise.

(and that's a broader argument than the one covered by the paper you link to)

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