Thanks for the recommendations, insights. I tried using glm.nb, but it didn't seem to like my data. I received the message (subscript) logical subscript too long. I am using the same dataframe as my previous glm. Do you know if I need to put the data in a different format?
Thanks, Wade On Wed, Apr 2, 2008 at 12:31 PM, Gavin Simpson <[EMAIL PROTECTED]> 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 > 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/<http://www.ucl.ac.uk/%7Eucfagls/> > UK. WC1E 6BT. [w] http://www.freshwaters.org.uk > %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% > > [[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.
