strange results from binomial lmer?
johnson4 at babel.ling.upenn.edu wrote:
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.
What version of the `lme4' package are you using? (Including the output of `sessionInfo()' would have helped here, as suggested by the posting guide.) Using version 0.999375-8 of the `lme4' package, I get > logLik(gs) # -1344 (df=3) 'log Lik.' -1344.320 (df=3) > logLik(g_s) # -1342 (df=3) 'log Lik.' -1341.794 (df=3) > logLik(s_s) # -1314 (df=3) 'log Lik.' -1314.395 (df=3) > logLik(gs_s) # -11823 (df=4) 'log Lik.' -1312.270 (df=4)
Could anyone advise me how to stop this from happening, and/or explain why it is?
You're likely using some obsolete version of `lme4'. Try downloading a development snapshot from R-Forge (http://r-forge.r-project.org/projects/lme4/). HTH, Henric
Thanks very much, Daniel
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