Effect Modification

Sander Greenland

The term effect modification has been applied to two distinct phenomena. Referring to the first phenomenon, effect modification simply means that some chosen measure of effect varies across levels of background variables. This phenomenon is more precisely termed effect-measure modification (Rothman & Greenland, 1998, chap. 4), and in the statistics literature is more often termed heterogeneity or “interaction,” Referring to the second phenomenon, effect modification means that the mechanism of effect differs with background variables, which is known in the biomedical literature as dependent action or (again) “interaction.” The two phenomena are sometimes confused, as reflected by the use of the same terms ( effect modification, interaction ) for both, but in fact they have only limited points of contact. To make the concepts and distinctions precise, suppose we are studying the effects that changes in a variable X will have on a subsequent variable Y, in the presence of ...

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