[R] How do I set the dispersion parameter in poisson glm?

[Achim Zeileis]

On Thu, 7 Oct 2010, Huso, Manuela wrote:

> Dear R users,
>
> 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?

The parameter estimation is invariant to the choice of dispersion 
parameter, hence you don't have to change anything in the glm() call 
above.

Only for the inference you have to set the dispersion, e.g.

summary(Tall.glm, dispersion = 2.5)

hth,
Z

> Many thanks in advance for your help,
>
> 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
>
> ______________________________________________
> 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.
>