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 over-dispersion. 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. HTH G > > Needless to say that I am at the outer limits of my statistical knowledge. > > Thanks for any help, > > Wade Wall > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. -- %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% 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@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
