I am trying to compare incidence rate ratios (IRR). This is what I have:
Group A (exposed vs. unexposed) IRR and
Group B (exposed vs. unexposed) IRR
So how would I correctly test for the difference between the IRR from Group A and the IRR from Group B? The question I would like to answer is whether the incremental differences observed in Group A are similar to those observed in Group B? Please help!
Best Answer
Generally speaking, you could either apply a Mantel-Haenszel test for whether the impact of the exposure factor differs between groups A and B (basically a stratified chi-squared test), or almost equivalently, model with Poisson regression and include an interaction term between exposure and group status to test whether these differ (EDIT: I mean whether exposure differs by group here too)
Wikipedia is (for once) not very verbose on this subject, but does give a two-liner summary. http://en.wikipedia.org/wiki/Cochran%E2%80%93Mantel%E2%80%93Haenszel_statistics