When does effect modification matter

When does effect modification matter?

MH methods assume that the true E - D odds ratio is the same in each stratum, an that the only reason for differences in the observed odds ratios between strata is sampling variation. We should check this assumption by applying the chi square test for heterogeneity, before reporting MH odds ratios, confidence intervals and P-values.

Kirkwood et al, 2003, Medical Statistics, Blackwell, p.187