[R] computation of dispersion parameter in quasi-poisson glm

[Achim Zeileis]

On Fri, 9 Apr 2010, Sven Garbade wrote:

> Hi list,
>
> can anybody point me to the trick how glm is computing the dispersion
> parameter in quasi-poisson regression, eg.
> glm(...,family="quasipoisson")?

It's the sum of squared Pearson residuals divided by the residual degrees 
of freedom. For example:

example("glm")
fm <- glm(counts ~ outcome + treatment, family = quasipoisson)
summary(fm)
sum(residuals(fm, type = "pearson")^2)/df.residual(fm)

hth,
Z

> Thanks &regards, Sven
>
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