**Simpson's paradox - Wikipedia**
https://en.wikipedia.org/wiki/Simpson%27s_paradox

Simpson's paradox, which goes by several names, is a phenomenon in probability and statistics, in which a trend appears in several different groups of data but disappears or reverses when these groups are combined.This result is often encountered in social-science and medical-science statistics and is particularly problematic when frequency data is unduly given causal interpretations.

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