Re: [R] Under-dispersion - a stats question?

From: Kjetil Holuerson <kjetil_at_redcotel.bo>
Date: Wed 12 Oct 2005 - 07:16:34 EST

Martin Henry H. Stevens wrote:
> Hello all:
> I frequently have glm models in which the residual variance is much
> lower than the residual degrees of freedom (e.g. Res.Dev=30.5, Res.DF
> = 82). Is it appropriate for me to use a quasipoisson error
> distribution and test it with an F distribution? It seems to me that
> I could stand to gain a much-reduced standard error if I let the
> procedure estimate my dispersion factor (which is what I assume the
> quasi- distributions do).
>

I did'nt see an answer to this. maybe you could treat as a quasimodel, but first you should ask why there is underdispersion.

Underdispersion could arise if you have dependent responses, for instance, competition (say, between plants) could produce underdispersion. Then you would be better off changing to an appropriate model. maybe you could post more about your experimental setup?

Kjetil

> Thank you for any input at all.
>
> Hank
>
> Dr. Martin Henry H. Stevens, Assistant Professor
> 338 Pearson Hall
> Botany Department
> Miami University
> Oxford, OH 45056
>
> Office: (513) 529-4206
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> http://www.cas.muohio.edu/~stevenmh/
> http://www.muohio.edu/ecology/
> http://www.muohio.edu/botany/
> "E Pluribus Unum"
>
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Received on Wed Oct 12 07:54:33 2005

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