R-alpha: offset in glm not working
Thomas Lumley
Thomas Lumley <thomas@biostat.washington.edu>
Fri, 17 Oct 1997 09:19:17 -0700 (PDT)
On Fri, 17 Oct 1997, Comblain muriel wrote:
> >
> > With the new version of R (septembre), the function glm seems to ignore
> the offset. The ouput is exactly the same with or without the offset
> specification. I tried the same program in the version of april and it
> worked corectly.
> >
Yes, this is a bug. The modelling code was changing quite rapidly at
about the time rsept was built, and it ended up without support for
offsets.
There are two problems. The version of model.frame/model.matrix in
rsept doesn't allow
glm(y~x+offset(z))
and the model.offset() function is broken so that
glm(y~x,offset=z)
doesn't work. The second problem is easy to fix
If you define
model.offset <- function (x)
{
offsets <- attr(attr(x, "terms"), "offset")
if (length(offsets) > 0) {
ans <- 0
for (i in offsets) ans <- ans + x[[i]]
}
else NULL
}
by adding it to your profile, or by editing the base library
then glm(y~x,offset=z) will work.
Thomas Lumley
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