glm: offset
HI, John, my understanding is that you should use log(...) instead of its original scale. Below is the logic in the case of poisson reg. log(y / offset) = x'b => log(y) - log(offset) = x'b => log(y) = x'b + log(offset) On Sun, Mar 2, 2008 at 10:01 PM, John Sorkin
<jsorkin at grecc.umaryland.edu> wrote:
R 2.6.0
Windows XP
A question about running a generalized linear model.
I am running a glm with
(1) a poisson distribution and a log link:
family=poisson(link = "log")
and an offset.
I would like to know if I should express the offset as the log of the offset value, i.e.
offset=log(NumUniqPt)
or as:
offset=NumUniqPt
I suspect I need to use the log, bu t I can't find any discussion of this in MASS 1994 or on the man page for glm.
Thanks
John
John Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
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