[R] poisson with overdispersion

Martin Hoyle plxmh at nottingham.ac.uk
Thu May 1 14:07:49 CEST 2003

Dear Amanda,
According to "Statistical Computing" by Crawley, p545, you can deal with overdispersion by any one of 3 ways;

1: Carry out significance tests using "F" rather than "Chi" in the anova function for comparison of deviances,
2: Use quasipoisson, and specify a variance function, or,
3: Use negative binomial error (see library(MASS), glm.nb).

To answer the second part of your qu, if you do summary(model), you will get the estimate  and s.e. of the log transform of your  response variable (log is the default link).


Martin Hoyle,
School of Life and Environmental Sciences,
University of Nottingham,
University Park,
NG7 2RD,
Webpage: http://myprofile.cos.com/martinhoyle

>>> Amanda Phipps <genepi51 at yahoo.com> 05/01/03 06:04am >>>
I am fitting a poisson model and it appears to have
overdispersion.  I am interested in using the
quasipoisson family (with the glm command).  Will this
account for the overdispersion in the model?  Is there
an additional method for accounting for a dispersion
parameter not equal to 1 (with the glm command)?  

And after fitting the model, how to I obtain the
fitted values and their respective standard errors?

Thank you-
Sarah Watson

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