You can't do a long term all-cause mortality study double blinded. It would never pass the Review Board for one thing. It would also be very expensive. And leav

You can't do a long term all-cause mortality study double blinded. It would never pass the Review Board for one thing. It would also be very expensive. And leaving the smallest problem for last, it's literally impossible to do a long term double blind study of "lifestyle". But the real purpose of review studies is to take a lot of result sets and see if the results from one study still hold. Of course this is susceptible to garbage-in, garbage-out, so you have to be aware of that when you evaluate the results - BUT - that's also true for individual studies, even pre-registered prospective double blinded studies (The Gold Standard). And yes, when you see an effect or outcome in a population, clinical studies often follow.