[R] Under-dispersion - a stats question?

[Jari Oksanen]

On Tue, 2005-10-11 at 17:16 -0400, Kjetil Holuerson wrote:
> Martin Henry H. Stevens wrote:
> > Hello all:
> > I frequently have glm models in which the residual variance is much  
> > lower than the residual degrees of freedom (e.g. Res.Dev=30.5, Res.DF  
> > = 82). Is it appropriate for me to use a quasipoisson error  
> > distribution and test it with an F distribution? It seems to me that  
> > I could stand to gain a much-reduced standard error if I let the  
> > procedure estimate my dispersion factor (which is what I assume the  
> > quasi- distributions do).
> > 
> 
> I did'nt see an answer to this. maybe you could treat as a
> quasimodel, but first you should ask why there is underdispersion.
> 
> Underdispersion could arise if you have dependent responses, for 
> instance, competition (say, between plants) could produce 
> underdispersion. Then you would be better off changing to an appropriate
> model. maybe you could post more about your experimental setup?
> 
Some ecologists from Bergen, Norway, suggest using quasipoisson with its
underdispersed residual error (while I wouldn't do that). However, it
indeed would be useful to know a bit more about the setup, like the type
of dependent variable. If the dependent variable happens to be the
number of species (like it's been in some papers by MHHS), this
certainly is *not* Poisson nor quasi-Poisson nor in the exponential
family, although it so often is modelled. I've often seen that species
richness (number of species -- or in R-speak 'tokens' -- in a
collection) is underdispersed to Poisson, and for a good reason. Even
there I'd play safe and use poisson() instead of underdispersed
quasipoisson(). 

cheers, jari oksanen
-- 
Jari Oksanen -- Dept Biology, Univ Oulu, 90014 Oulu, Finland
Ph. +358 8 5531526, cell +358 40 5136529, fax +358 8 5531061
email jari.oksanen at oulu.fi, homepage http://cc.oulu.fi/~jarioksa/