> r2 <- glmer(sa~(1|id), data=sexpartner, family=poisson(), nAGQ=6)
Error in mer_finalize(ans) : Downdated X'X is not positive definite, 1.
Running a closely related (specifically, the outcome variables match on
missings)
> r <- glmer(pAnyUAVI~pIsMale+pEthnic+(1|studyidx), data=sexpartner,
family=binomial, nAGQ=6)
produces no error.
I trimmed the covariates until only the intercept was left. I used id
<- factor(studyidx) in case the fact that studyidx was a character was a
problem (the archive had a statement that it needed to be a factor).
The original count data had a minimum of 1; sa was the count -1 so that
it would look more as expected. At every step along the way I got the
same error.
My searches suggested the message usually indicates a colinearity
problem, but I can't see how it could be, or why it wouldn't have shown
up for the binomial as well as the poisson.
My data are unusual; cluster size varies widely and there are lots of
singleton clusters. I tried simulating data with lots of singletons;
the poisson fit them fine. As mentioned the counts have no zeros
(though the sa outcome above does).
Can anyone suggest steps to diagnose or fix the problem?
R 2.15.2; lme4 Version: 0.999999-0, Date: 2012-06-22 from CRAN.
Ross Boylan
P.S. Would extending the model to a poisson distribution conditional
on the value being greater than 0 be hard?