I am trying to compare the incidence rates among 3 groups using exact Poisson regression(the sample size is small, and events are rare). My code is as below:
proc genmod data = data; class B/param=glm ref=first; model A = B/dist=poisson link=log offset=log_PY ; exact B; run;
where A is the event indicator, B is a 3-level categorical variable, and log_PY is log(person-years).
I got a message "Note: # indicates that the conditional distribution is degenerate.", and the exact estimates were not produced. The Exact Conditional Tests looks like below:
Exact Conditional Tests
Effect Test Statistic p-Value Exact Mid
B Score . # . .
Probability 1.0000 # 1.0000 0.5000
I am new to this and wonder if anyone could help me to understand why I got this kind of outputs and how to solve the problem?
If any other SAS proc or R package can do the job, please let me know.
Thanks a lot!
