Test for Interaction in a Measure of Association

(A) Set up null and alternative hypotheses

H0: MA1 = MA2  = . . . = MAG (homogeneity of measures of association, indicating no interaction)
H1: at least one MAi differs (indicating interaction)

where MAi is a measure of association, either a strata-specific RR or OR

(B) Set alpha level

(C) Test statistic is a chi-square test for interaction (separate chi-square interaction statistics for relative risks and odds ratios). The p value is the probability region in right tail of Chi-square with G - 1 degrees of freedom, where G represents the number of strata tested. This statistic is computed with the TABLES E D C command.

(D) Conclusion: Reject H0 if p < alpha.  A rejected null hypothesis indicates that the observed heterogeneity of effect (interaction) is significant.