# [Rd] help page for anova.glm/variation between S-PLUS and R behavior

**Ben Bolker
**
bolker@zoo.ufl.edu

*Mon Jan 27 18:50:04 2003*

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 bolker@zoo.ufl.edu
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