# Comparing Incidence Rate Ratios

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!

• Sec. 6.3 of Agresti 2002 (2nd ed.) discusses some options, including the CMH test mentioned by @JamesStanley If you are an R user you may find the R/S companion to this book helpful, specifically p.102 sec E. The R function mantelhaen.test runs the CMH test. – conjugateprior Feb 1 '13 at 11:42
• Oops. That was more of an answer than I was intending. Perhaps @James-Stanley wants to incorporate it into his. – conjugateprior Feb 1 '13 at 11:44