sas >> OFFSET variable in proc genmod

by MH » Fri, 27 Nov 2009 01:49:43 GMT


I am wondering which option would be more appropriate to account for
days in community ( vs. in jail) when examining days of drug use in
two different study groups. The options are either log of community
days as offset variable or a ratio of days of drug use by community
This is zero inflated count data, and I am using proc genmod. Results
are slightly different when I use these two options. I am not sure

Has anyone had similar issue? I would appreciate any help.


sas >> OFFSET variable in proc genmod

by stringplayer_2 » Tue, 01 Dec 2009 04:22:58 GMT

Which model is preferred using an AIC or SBC criterion? I would
prefer to interpret the model which has the better information
criterion values.


Dale McLerran
Fred Hutchinson Cancer Research Center
Ph: (206) 667-2926
Fax: (206) 667-5977

sas >> OFFSET variable in proc genmod

by Oliver Kuss » Tue, 01 Dec 2009 16:23:33 GMT

Dear MH,
I am surprised that you are getting different results, when (1) using
the ratio as the outcome or (2) using log(denominator) as an offset.
Both models are equivalent in mathematical sense and should give
exactly equal results (at least when you use the Poisson distribution
and a log link). This can also be seen from the little toy example

Hope that helps,

data poisson;
input numerator denominator covariate;
3 100 1
4 200 1
6 150 0
8 180 0

proc genmod data=poisson;
model numerator/denominator=covariate / d=poisson link=log;

proc genmod data=poisson;
model numerator=covariate / d=poisson link=log

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