[R] AIC for quasipoisson link

Marc Schwartz marc_schwartz at comcast.net
Fri Oct 31 20:29:45 CET 2008

on 10/31/2008 01:07 PM Antonio.Gasparrini at lshtm.ac.uk wrote:
> Dear fellows,
> I'm trying to extract the AIC statistic from a GLM model with quasipoisson link.
> The formula I'm referring to is 
> AIC = -2(maximum loglik) + 2df * phi
> with phi the overdispersion parameter, as reported in:
> Peng et al., Model choice in time series studies os air pollution and mortality. J R Stat Soc A, 2006; 162: pag 190.
> Unfortunately, the function logLik doesn't work for a quasipoisson link.
> Do you know a fast method to extract the AIC for these models?
> Thanks in advance

I was under the impression that there is no log likelihood for quasi*
family models, thus no AIC, which is why they are not calculated/printed
in the glm() summary outputs.

If you want to model overdispersed data and need the AIC, you should
look at glm.nb() in MASS for a negative binomial model:


This would also avail you of the anova.glm() methods for comparing
models, which the quasi* families would not.

You might also want to look at:


which is the vignette from the pscl package.


Marc Schwartz

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