This supplements Noah Stein's answer. Judea Pearl has written extensively on causal models, both in his book *[Causality][1]*, and in individual papers, such as "Causal Diagrams for Empirical Research," *Biometrika*, 82(4), 669-710, 1995, from which this figure is taken: <br /> ![Fig1][2] [1]: http://bayes.cs.ucla.edu/BOOK-2K/ [2]: http://cs.smith.edu/~orourke/MathOverflow/PearlFig1.jpg