Effect Modification and Interaction

Sander Greenland

The term effect modification has been applied to two distinct phenomena. For the first phenomenon, effect modification simply means that some chosen measure of effect varies across levels of background variables. This phenomenon is thus more precisely termed effect-measure modification , and in the statistics literature it 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 often confused, as reflected by the use of the same terms (effect modification and interaction) for both. 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 a background variable Z ...

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