[S-News] Re: fitting offsets

Alan Zaslavsky (zaslavsk@hcp.med.harvard.edu)
Wed, 4 Feb 1998 07:14:16 -0500 (EST)


> From: Andrew McCulloch <andrew.mcculloch@lshtm.ac.uk>
> I would like to fit an offset in a poisson regression. Can
> someone tell me how I do this in general. I was thinking of using the
> I() operator and adding the offset to the constant but it would be
> good to know how this is done generally in Splus model formula.

You should include an offset() term in the model expression. Actually,
the I() approach would not work because all that I() does is require that
the contents of the () be evaluated arithmetically rather than in model
language; the resulting term is still entered as a model variable for
which a coefficient is fitted.
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