[R] AIC in glm.nb and glm(...family=negative.binomial(.))

[Prof Brian Ripley]

You have the code, so take a look for youself.  There's an error in the 
aic formula in negative.binomial (and neg.bin)

On Tue, 15 Jun 2004, Ken Knoblauch wrote:

> Can anyone explain to me why the AIC values are so different when
> using glm.nb and glm with a negative.binomial family, from the MASS
> library?  I'm using R 1.8.1 with Mac 0S 10.3.4.
> 
> >library(MASS)
> > dfr <- data.frame(c=rnbinom(100,size=2,mu=rep(c(10,20,100,1000),rep(25,4))),
> +                   f=factor(rep(seq(1,4),rep(25,4))))
> > AIC(nb1 <- glm.nb(c~f, data=dfr))
> [1] 1047
> > AIC(glm(c~f, family=negative.binomial(nb1$theta), data=dfr))
> [1] -431804
> 
> Actually, the difference is already apparent with the function logLik,
> but I still would like to understand the difference in what is
> calculated in the two instances.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595