No. Binomial data can indeed be overdispersed. See McCullagh & Nelder (1989, section 4.5). Accounting for over(under)dispersion in binomial and Poisson distributions is, in fact, one of the original impetus for GEE type developments. See also a nice paper by Liang & McCullagh (Biometrics 1993, p. 623-630), which discusses numerous examples of overdispersion in binary data.
Ravi. ---------------------------------------------------------------------------- ------- Ravi Varadhan, Ph.D. Assistant Professor, The Center on Aging and Health Division of Geriatric Medicine and Gerontology Johns Hopkins University Ph: (410) 502-2619 Fax: (410) 614-9625 Email: [EMAIL PROTECTED] Webpage: http://www.jhsph.edu/agingandhealth/People/Faculty/Varadhan.html ---------------------------------------------------------------------------- -------- -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Gavin Simpson Sent: Monday, February 11, 2008 12:37 PM To: anna banana Cc: r-help@r-project.org Subject: Re: [R] overdispersion + GAM On Mon, 2008-02-11 at 07:35 -0800, anna banana wrote: > Hi, > > there are a lot of messages dealing with overdispersion, but I couldn't find > anything about how to test for overdispersion. I applied a GAM with binomial > distribution on my presence/absence data, and would like to check for > overdispersion. Does anyone know the command? Bernoulli data (presence/absence of single species say) can't be overdispersed, so there is no need to test or correct for it. G > > Many thanks, > > Anna -- %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% 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. ______________________________________________ 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.
