Dear Jeroen, drop1() respects relationships of marginality among the terms in a model and won't drop a lower-order term (e.g., a main effect) when a higher-order relative (e.g., an interaction to which the main effect is marginal) is in the model. If you want a complete "type-III" ANOVA table and are careful with contrast coding (which you were not, since contr.treatment produces contrasts that are not orthogonal in the row basis of the model matrix -- you could use, e.g., contr.sum), then you can use the Anova() function in the car package, specifying type="III".
Note: I'm responding to this message on the r-devel list where it was posted, but this is really a question for r-help. I hope this helps, John ------------------------------ John Fox, Professor Department of Sociology McMaster University Hamilton, Ontario, Canada web: socserv.mcmaster.ca/jfox > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On > Behalf Of Jeroen Ooms > Sent: September-30-08 9:28 AM > To: r-devel@r-project.org > Subject: [Rd] weird behavior of drop1() for polr models (MASS) > > > I would like to do a SS type III analysis on a proportional odds logistic > regression model. I use drop1(), but dropterm() shows the same behaviour. It > works as expected for regular main effects models, however when the model > includes an interaction effect it seems to have problems with matching the > parameters to the predictor terms. An example: > > library("MASS"); > options(contrasts = c("contr.treatment", "contr.poly")); > > house.plr1 <- polr(Sat ~ Infl + Type + Cont, weights = Freq, data = > housing); > drop1(house.plr1,attributes(house.plr1$terms)$term.labels,test="Chisq"); > > house.plr2 <- polr(Sat ~ Infl * Type + Cont, weights = Freq, data = > housing); > drop1(house.plr2,attributes(house.plr2$terms)$term.labels,test="Chisq"); > > Notice that model 2 has a * instead of a + between predictors Infl and Type. > In model 1, estimated parameters are nicely attributed to the right term, > however in house.plr2, only 2 of the 4 terms are evaluated. > > I am using R version 2.7.2 (2008-08-25) i386-pc-mingw32, and MASS_7.2-44. > > -- > View this message in context: http://www.nabble.com/weird-behavior-of- > drop1%28%29-for-polr-models-%28MASS%29-tp19742120p19742120.html > Sent from the R devel mailing list archive at Nabble.com. > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
