Good for women, good for men, bad for people: Simpson's paradox and the importance of sex-specific analysis in observational studies.
Even if a medial intervention has a beneficial effect in both men and women, an observational study that combines data from men and women can lead to the incorrect conclusion that treatment has a harmful effect. This is an example of Simpson's paradox, which although uncommon in practice, does, in fact, occur (Wainer H. Simpson's paradox. Chance 1999;12:43). More importantly, it is likely that in an observational study, a related result will occur; namely, ignoring sex in the analysis will lead to biased results. To better understand why Simpson's paradox and the related result occur, we present a graphic explanation.
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Division of Cancer Prevention, National Cancer Institute, EPN 3131, 6130 Executive Boulevard, MSC 7354, Bethesda, MD 20892-7354, USA.
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D015982:Bias; D005260:Female; D006801:Humans; D008297:Male; D016032:Randomized Controlled Trials as Topic; D012727:Sex Characteristics; D016019:Survival Analysis