Effect Modification

1.  Effect modification is present when the size of an association differs by another factor. 2.  The presence of effect modification can suggest synergy between exposure variables. 3.  Effect modification is a different concept from confounding; a particular characteristic can function as a confounder, an effect modifier, both, or neither in a given study. 4.  The likelihood ratio test evaluates whether the size of an association is statistically different across two or more categories of another factor. 5.  The p-value from the likelihood ratio test represents the probability of finding the observed difference in the size of an association across subgroups due to chance.