Body & HealthMental model
Why vitamin pills work in observations but not in trials.
People who voluntarily take vitamins usually also exercise more, smoke less and see doctors more often. In observational studies the supplement then looks healthy. Randomised trials, where chance decides who gets it, found no benefit for many vitamin supplements in healthy people. The difference was the person, not the pill.
Useful: Observation shows who does something – only an experiment shows what it does.
Source: en.wikipedia.org