[R] glm and overdispersion

John Fox jfox at mcmaster.ca
Tue Feb 18 13:55:03 CET 2003

Dear Alexandre,

The Anova function in the car package by default will calculate F-tests 
(specified by test="F") using a dispersion estimate based on the Pearson 
residuals. See ?Anova for details.

I hope that this helps,

At 12:04 PM 2/18/2003 +0100, Hardouin Loïc wrote:

>I am performing glm with binomial family and my data show slight
>overdispersion (HF<1.5). Nevertheless, in order to take into account for
>this heterogeneity though weak, I use F-test rather than Chi-square
>(Krackow & Tkadlec, 2001). But surprisingly, outputs of this two tests
>are exactly similar. What is the reason and how can I scale the output
>by overdispersion ??
>Thank you,
>Alexandre MILLON

John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox

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