I'm thinking of Hawthorne effect or such. Following people for so many years will have SOME effect on their health behavior, knowing they're examined every two years.
They still developed severe diseases, even though they knew their health was being observed. The study showed the risk of smoking etc. which were not really known before so of course the intensity of the effects might be a bit smaller but that is not really relevant when it comes to the impilication of the findings, right? :)
Four minutes into this discussion a study flaw comes to mind: They selected out young people reaching thirty who may have already been developing CAD, so the cohort they studied, assuming on average they all ate a similar American diet, were preselected to be possibly more resistant to CAD, as they all didn't have CAD by age 30...
