Simpson's Paradox: A Graphical Treatment
It is possible for a pair of dichotomous, categorical variables to have an overall positive association (say) in a stratified population, and at the same time be negatively associated in every stratum. This (essentially) is Simpson's paradox. A graphical device, the risk diagram, provides insight into Simpson's paradox and related concepts.
Redfearn, William J., "Simpson's Paradox: A Graphical Treatment" (November 1989). U.C. Berkeley Division of Biostatistics Working Paper Series. Working Paper 9.
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