[R] Better idea than Poisson?

[Jim Lindsey]

> 
> On Fri, 25 Jan 2002, Patrick Connolly wrote:
> 
> >
> > My data has three factors, a discrete response and an offset column.
> > >From the summary of a glm object derived using three way interactions,
> > I get this deviance information:
> >
> > (Dispersion parameter for poisson family taken to be 1)
> >
> >     Null deviance: 39244  on 896  degrees of freedom
> > Residual deviance: 11913  on 795  degrees of freedom
> > AIC: 13905
> >
> > Number of Fisher Scoring iterations: 6
> >
> >
> > The rather high residual deviance could indicate that the data does
> > not fit a poisson distrubution very well.  That's believable since in
> > quite a number of combinations of the three factors, zero occurs more
> > often that one might expect for a Poisson distribution.
> >
> >
> > Are there suggestions as to what might be a better way to analyse this
> > data?
> 
> I would at least try a negative binomial (glm.nb in MASS), but as that
> seems to be a very large degree of over-dispersion it might not fit much
> better.  You could fairly easily write code for a zero-augmented Poisson
> (a mixture of a Poisson and always zero) by a direct likelihood
> maximization.

The zero-augmented Poisson regression is available in my fmr function,
and a number of different overdispersed binomial distribution
regressions, including negative binomial, are available in gnlr, all
in my gnlm library at www.luc.ac.be/~jlindsey/rcode.html
  Jim

> 
> -- 
> 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 272860 (secr)
> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
> 
> -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
> r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
> Send "info", "help", or "[un]subscribe"
> (in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
> _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._
> 

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._