[R] a bug in the offset parameter syntax in the geepack package?

ahnven ahnven at yahoo.it
Wed Aug 19 18:07:51 CEST 2009

Dear R-users,

I was doing some Poisson regression using the geepack package and I found a
"strange" behaviour of the geeglm command when defining an offset. Maybe
it's my limited knowledge of R syntax, or maybe it's something else.

Let's make an example. After loading the geepack library, you may write


which is equivalent to


however, the command works also with the syntax


giving different (wrong) results!

I discovered it because I was calculating some incidence rates by hand and
then comparing with the glm estimates. In the manual, indeed, the way in
which the offset should be written is not so easy to understand.

Thanks for any response,
Mattia Prosperi

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