It is true that correlation does not equal causation, but that does not mean we are powerless about it either. To understand how to attack this kind of problem, we need to recall a few important (but pretty rarely discussed) principles of science:
- X causes Y.
- Y causes X.
- Some third variable, W, causes both X and Y.
Although it is true that we cannot conclude that X causes Y directly from a correlation, we can come to that conclusion indirectly, using a process of elimination to show that the other two possibilities are inferior (less parsimonious) explanations for the correlation. This is best shown by concrete example: