Re: [R] glm: offset

From: Wensui Liu <liuwensui_at_gmail.com>
Date: Sun, 02 Mar 2008 22:19:01 -0500

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
> Baltimore VA Medical Center
> 10 North Greene Street
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
> (Phone) 410-605-7119
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-- 
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WenSui Liu
ChoicePoint Precision Marketing
Phone: 678-893-9457
Email : wensui.liu_at_choicepoint.com
Blog   : statcompute.spaces.live.com

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Received on Mon 03 Mar 2008 - 03:22:08 GMT

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