Correlation does not imply causation

Correlation does not imply causation
The search for causation is central to epidemiology, and all sciences. Often causation is not directly evident from available data. Correlation then is crucial to any consideration of causation. However, correlation, while necessary, is not sufficient for establishing causation. Granted, as Tufte pointed out, it often is a good hint. Qualitative, weight-of-evidence considerations as those involving Hill-like criteria (e.g., plausibility, temporality, specificity, coherence with existing theory) must also be applied.

Just distinguishing causation from correlation can be problematic. To say an event (cause) produces a result (effect) is circular containing definitional dependence. ‘Correlation’ and ‘causation’ are defined at different levels of abstraction. Correlation is often established by statistical similarity, as two variables representing events changing together with unlikely-by-chance regularity. While ‘causation’ has logical and philosophical considerations often referenced to a counterfactual.

For randomized experiments and clinical trials, correlation may generally be closer to causation than in observational studies, with less rigorously generated observations. Subject equality as through randomization helps in eliminating meaningless associations. However, caution is always a caveat to claiming causation. Examples of nonsensical associations abound (e.g., the increase in autism is associated with the increase in home schooling.)