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

From: Jeroen Ooms <j.c.l.ooms_at_uu.nl>
Date: Tue, 30 Sep 2008 06:28:14 -0700 (PDT)

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.

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