[R] strange results from binomial lmer?

From: <johnson4_at_babel.ling.upenn.edu>
Date: Thu, 13 Mar 2008 19:52:02 -0400


I'm running lmer repeatedly on artificial data with two fixed factors (called 'gender' and 'stress') and one random factor ('speaker'). Gender is a between-speaker variable, stress is a within-speaker variable, if that matters. Each dataset has 100 rows from each of 20 speakers, 2000 rows in all.

About 5% of the time I get a strange result, where the lmer() model with BOTH fixed factors and the random factor ('gs_s') comes out MUCH worse compared to the models with ONE fixed factor and the random factor ('g_s' and 's_s'), and also compared to the glm() model with both fixed factors and no random factor ('gs').

This doesn't make much sense to me.

I've placed a dataset on the Web that exhibits this behavior, as follows:

dat <- read.csv("http://www.ling.upenn.edu/~johnson4/strange.csv")

gs <- glm(outcome~gender+stress,binomial,dat) g_s <- lmer(outcome~gender+(1|speaker),dat,binomial) s_s <- lmer(outcome~stress+(1|speaker),dat,binomial) gs_s <- lmer(outcome~gender+stress+(1|speaker),dat,binomial)

logLik(gs)         #  -1344 (df=3)
logLik(g_s)        #  -1342 (df=3)
logLik(s_s)        #  -1314 (df=3)
logLik(gs_s)       # -11823 (df=4)

This seems like an error of some kind. The glm() model with both fixed effects is well-behaved, but lmer() seems to be going haywire when confronted with the same situation plus the random effect.

Could anyone advise me how to stop this from happening, and/or explain why it is?

Thanks very much,
Daniel



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