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SUMMARY We consider the collapsibility of relative risks in contingency tables with a polytomous response variable. Simple collapsibility of relative risks means that they are consistent over all levels of a collapsed variable and also coincide with the corresponding marginal relative risk. Strong collapsibility of relative risks means that they retain simple collapsibility no matter how the variable is partially collapsed. We present necessary and sufficient conditions for simple and strong collapsibilities of relative risks.
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Zhi Geng (1992) studied this question.
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