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1thenumber t1_jbc5c0m wrote

Epidemiology cannot speak to causation, so you are jumping too far ahead. It can generate a hypothesis that can then be tested in a controlled trial. But asking people to self-report on habits over a long period is not going to tell us cause; it is going to tell us what every similar study does - that healthy people are healthy, and unhealthy people are unhealthy.

The healthy group in this case might be simply healthy because they care about being healthy - they are "adherers" or "compliers". There's a really fascinating study that was done in 1980 on a cholesterol drug called clofibrate, where the initial results were unspectacular when compared to placebo. The clofibrate showed no improvement over the placebo group in terms of mortality.

However, when those two groups were split again between "adherers" and non-adherers, the adherers in BOTH groups saw almost the exact same reduction in mortality that the mouthwash study saw - about 50%. This means in a placebo group, where no actual intervention was being taken, simply adhering to the instructions given by the study was enough to almost halve your mortality risk.