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
(Fax) 410-605-7913 (Please call phone number above prior to faxing)
Confidentiality Statement:
This email message, including any attachments, is for th...{{dropped:6}}
Yes, use the log. I've had the same problem in the past, too. Try it on a toy example to confirm it for yourself. Cheers, Simon. On Sun, 2008-03-02 at 22:01 -0500, John Sorkin 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 > (Fax) 410-605-7913 (Please call phone number above prior to faxing) > > Confidentiality Statement: > This email message, including any attachments, is for th...{{dropped:6}} > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code.-- Simon Blomberg, BSc (Hons), PhD, MAppStat. Lecturer and Consultant Statistician Faculty of Biological and Chemical Sciences The University of Queensland St. Lucia Queensland 4072 Australia Room 320 Goddard Building (8) T: +61 7 3365 2506 http://www.uq.edu.au/~uqsblomb email: S.Blomberg1_at_uq.edu.au Policies: 1. I will NOT analyse your data for you. 2. Your deadline is your problem. The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data. - John Tukey.
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 > (Fax) 410-605-7913 (Please call phone number above prior to faxing) > > Confidentiality Statement: > This email message, including any attachments, is for th...{{dropped:6}} > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. >-- ==============================WenSui Liu ChoicePoint Precision Marketing Phone: 678-893-9457 Email : wensui.liu at choicepoint.com Blog : statcompute.spaces.live.com