 This phenomenon was first pointed out in papers by Karl G. Pearson (1899) and George U. Yule (1903), but it was Simpson’s short paper “The interpretation of interaction in contingency tables” (1951), discussing the interpretation of such association reversals, that led to the phenomenon being labeled as “Simpson’s Paradox”. The phenomenon is, however, broader than independence in the overall population and positive association in the subpopulations; for example, the associations may also be reversed. Nagel and Cohen (1934: ch. 16) provide an example of such a reversal as part of a exercise for logic students.