When using test="F" in stat.anova() / anova.glm(), R uses the assumed dispersion parameter for the specified family (e.g. scale=1 for binomial), while S-PLUS automatically uses the estimated dispersion parameter (residual deviance/residual df). I think there are good reasons for the behavior in R -- it fits with the "you get what you actually asked for" philosophy -- and there are hints in the documentation (anova.glm(): "for [models] with dispersion estimated by moments ... the F test is most appropriate"; family/binomial(): "The behaviour of S-PLUS is closer to the quasi- variants"). Would R-core consider it reasonable to add yet another sentence to the anova.glm() documentation, e.g. after the "... F test is most appropriate" sentence above, adding: "(Note: unlike R, S-PLUS automatically uses the moment-estimated dispersion for F-tests even for the binomial and Poisson families: see ?quasibinomial.)" ?
Ben Bolker -- 318 Carr Hall [EMAIL PROTECTED] Zoology Department, University of Florida http://www.zoo.ufl.edu/bolker Box 118525 (ph) 352-392-5697 Gainesville, FL 32611-8525 (fax) 352-392-3704 ______________________________________________ [EMAIL PROTECTED] mailing list http://www.stat.math.ethz.ch/mailman/listinfo/r-devel
