# [R] fixing the dispersion parameter in glm

Huso, Manuela manuela.huso at oregonstate.edu
Wed Sep 29 01:03:13 CEST 2010

```I would like to fit a glm with Poisson distribution and log link with a known dispersion parameter.  I do not want to estimate the dispersion parameter.  I know what it is, so I simply want to fix it at a constant for this and other models to follow.  My simple, no covariate model is:

Tall.glm<-glm(Seedling~1,
family=poisson,
offset(log(area)),
data=tallPSME.df)

I want to fix the dispersion parameter at 2.5.  How can I do this, please?

Manuela

>::<>::<>::<>::<>::<>::<>::<>::<>::<
Manuela Huso
Consulting Statistician
201H Richardson Hall
Department of Forest Ecosystems and Society
Oregon State University
Corvallis, OR   97331
ph: 541-737-6232
fx: 541-737-1393

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of L Brown
Sent: Tuesday, September 28, 2010 2:47 PM
To: r-help at r-project.org
Subject: [R] drawing samples based on a matching variable

Hi, everyone. I have what I hope will be a simple coding question. It seems
this is a common job, but so far I've had trouble finding the answer in
searches.

I have two matrices (x and y) with a different number of observations in
each. I need to draw a random sample without replacement of observations
from x, and then, using a matching variable, draw a sample of equal size
from y. It is the matching variable that is hanging me up.

For example--

> # example matrices. lets assume seed always equals 1. (lets also assume I
have assigned variable names A and B to my columns..)
> set.seed(1)
> x<-cbind(1:10,sample(1:5,10,rep=T))
> x
[A] [B]
[1,]    1    2
[2,]    2    2
[3,]    3    3
[4,]    4    5
[5,]    5    2
[6,]    6    5
[7,]    7    5
[8,]    8    4
[9,]    9    4
[10,]   10    1

> y<-cbind(1:14,sample(1:5,14,rep=T))
> y
[A] [B]
[1,]    1    2
[2,]    2    2
[3,]    3    3
[4,]    4    5
[5,]    5    2
[6,]    6    5
[7,]    7    5
[8,]    8    4
[9,]    9    4
[10,]   10    1
[11,]   11    2
[12,]   12    1
[13,]   13    4
[14,]   14    2

> #draw random sample of n=4 without replacement from matrix x.
> x.samp<-x[sample(10,4,replace=F),]
> x.samp
[A] [B]
[1,]    3    3
[2,]    4    5
[3,]    5    2
[4,]    7    5

Next, I would need to draw four observations from matrix y (without
replacement) so that the distribution of y\$B is identical to x.samp\$B.

I'd appreciate any help, and sorry to post such a basic question!

LB

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