[R] Overdispersion in count data
Achim.Zeileis at wu-wien.ac.at
Wed Apr 2 19:40:09 CEST 2008
On Wed, 2 Apr 2008, Gavin Simpson wrote:
> On Wed, 2008-04-02 at 12:03 -0400, Wade Wall wrote:
> > Hi all,
> > I have count data (number of flowering individuals plus total number of
> > individuals) across 24 sites and 3 treatments (time since last burn).
> > Following recommendations in the R Book, I used a glm with the model y~
> > burn, with y being two columns (flowering, not flowering) and burn the time
> > (category) since burn. However, the residual deviance is roughly 10 times
> > the number of degrees of freedom, and using the quasibinomial distribution
> > doesn't change this. Any suggestions as to why the quasibinomial
> > distribution doesn't change the residual deviance and how I should proceed.
> > I know that this level of residual deviance is unacceptable, but not sure is
> > transformations are in order.
> The quasi families estimate the dispersion parameter rather than assume
> it is fixed. This doesn't change the estimates for the coefficients, but
> it may change their standard errors if the estimated dispersion
> parameter is different from 1, and hence the test statistics and their
> p-values. As such the residual deviance doesn't change, you are just
> adjusting the interpretation of coefficients to take account of the
> If you are not happy with the fitted model there are numerous options
> you could try, including fitting a negative binomial (NB) GLM (see
> glm.nb() in package MASS) or a zero-inflated Poisson or NB model or a
> Hurdle model. Functions to fit the ZIP/ZINB or Hurdle models can be
> found in the pscl package.
The pscl also provides
vignette("countreg", package = "pscl")
which discusses Poisson, quasi-Poisson, negative binomial models and
zero-augmented versions of these.
> > Needless to say that I am at the outer limits of my statistical knowledge.
> > Thanks for any help,
> > Wade Wall
> > [[alternative HTML version deleted]]
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> Dr. Gavin Simpson [t] +44 (0)20 7679 0522
> ECRC, UCL Geography, [f] +44 (0)20 7679 0565
> Pearson Building, [e] gavin.simpsonATNOSPAMucl.ac.uk
> Gower Street, London [w] http://www.ucl.ac.uk/~ucfagls/
> UK. WC1E 6BT. [w] http://www.freshwaters.org.uk
> R-help at r-project.org mailing list
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
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