Simpson’s paradox, a tale of causality

Abstract

For the mathematically wary and unwary alike, Simpson’s paradox may well function as a permanent invitation to error. We present Simpson’s paradox and discuss its nature based on three examples. It appears that to run afoul of Simpson’s paradox it suffices to (a) conflate an invalid probabilistic reasoning with a valid instance of unassailable causal reasoning, or (b) confuse the evidential concept of learning from observation, which for rational agents proceeds by conditioning on the evidence, with the causal concept of acting, represented in causal analysis by the operation of intervening in a causal graph.