Do men who shave daily have fewer heart attacks than those who only shave once every two or three days? This is the finding of at least one study. However, Michael R. Eades, M.D. uses this example to show people the difference between observational scientific studies that generate hypotheses and causal scientific studies that generate reproducible scientific facts.
The shaving study is an example of an observational study. When you dig deeper, as Eades does, you see that it is not shaving which is linked to fewer heart attacks, but the attributes of men who shave more often that lead to fewer heart attacks. So, says he, correlation is not causation. This is a great lesson to remember, the next time a scientific study du jour is released that makes some claim. If it was an observational study, it merely generated an hypothesis, which needs further testing.