This. So many studies fail to even mention this now. Next time you see a study on the health benefits of some new type of exercise for example, it's worth check

This. So many studies fail to even mention this now. Next time you see a study on the health benefits of some new type of exercise for example, it's worth checking that the results aren't explained by the healthy group simply being more inclined to exercise (see also: less sleep leads to a shorter life, anxiety leads to health problems, etc)