Such studies can identify key factors when the signal/noise ratio is strong enough, but are very sensitive to selection bias, and prone to the existence of confounding factors. Lastly, ecological studies may be used (where the global environment variables and their global effect on two different populations are compared). Such studies are "cheap and dirty": they can be easily conducted on very large populations (the whole USA, in Dr Cohen's study), but are prone to the existence of confounding factors, and exposed to the ecological fallacy problem.