According to the philosopher John Stuart Mill:
- The cause (independent variable) must precede the effect (dependent variable) in time.
- The two variables are empirically correlated with one another.
- The observed empirical correlation between the two variables cannot be due to the influence of a third variable that causes the two under consideration.
Correlation does not prove causation!!
Shadish, W., Cook, T. & Campbell, D (2002). Experimental & Quasi- Experimental Designs for Generalized Causal Inference. Boston: Houghton Mifflin.