[Rd] weird behavior of drop1() for polr models (MASS)

Jeroen Ooms j.c.l.ooms at uu.nl
Tue Sep 30 15:28:14 CEST 2008

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:

options(contrasts = c("contr.treatment", "contr.poly"));

house.plr1 <- polr(Sat ~ Infl + Type + Cont, weights = Freq, data =

house.plr2 <- polr(Sat ~ Infl * Type + Cont, weights = Freq, data =

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.

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