Causality and correlation

This article is a good reminder that you need a link. And it also includes my favorite xkcd strip of all time.

“Omitted Variable Bias“, a type of statistical bias that illustrates this problem. Suppose we see from Department of Defense data that male U.S. soldiers are more likely to be killed in action than female soldiers... So there is a correlation between the gender of soldiers and the likelihood of being killed in action. One could – and one often does – conclude from such a finding that there is a causation of some kind: the gender of soldiers increases the chances of being killed in action... However, it’s here that the Omitted Variable Bias pops up. The real cause of the discrepancy between male and female combat mortality may not be gender or a gender related thing, but a third element, an “omitted variable” which doesn’t show in the correlation. More...