Again, thanks @PG. Sorry for having so many questions. Your simulation is very helpful. However, if one wants to pick an estimate, which one should he use? It seems that using "Offset" gives pretty similar "estimate" as "Rate" does, although the SEs are different. But if using "Rate*Mean(days)", it gives completely different "estimate" but similar SEs. I also tried my real data using PROC GENMOD on both Offset model and Rate model. It seems that the two give pretty close results in terms of "estimate" and "SE". Here are my code snippets: 1. Poisson model with offset proc genmod data=final;
title "BV-003 Part 2 - Poisson Model";
class ptno treat;
model numcr = treat /type3 dist=poi offset=logdayct;
repeated subject=ptno;
estimate 'D vs A' treat -1 1 0/exp;
estimate 'E vs A' treat -1 0 1/exp;
estimate 'D vs E' treat 0 1 -1/exp;
run; 2. Poisson model with rate data: proc genmod data=final;
title "BV-003 Part 2 - Poisson Model on the Cramp/Day Ratio";
class ptno treat;
model crp_day = treat /type3 dist=poi;
repeated subject=ptno;
estimate 'D vs A' treat -1 1 0/exp;
estimate 'E vs A' treat -1 0 1/exp;
estimate 'E vs D' treat 0 1 -1/exp;
run; And here are the outputs: For 1, For 2,
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