[R-sig-ME] Singular estimated var-cov

[francois.mercier at novartis.com]

Dear list members,

I try to fit a model (using lmer) to data recorded at 4 time points 
(days). Each such time series corresponds to a distinct subject. There are 
two treatment groups. There is also a patient-level covariate ("o" or 
"b").  I am attaching the data frame (as a binary R object) and the R 
script that loads the data frame and fits the models.

The questions are 1) whether the drug effect is influenced by the 
covariate, and 2) whether there is a temporal trend in drug effect over 
days.

The problem is that according LMER the covariance matrix for this problem 
is singular, and as a result the fitted models do not capture the 
variability of slopes that is seen in the data. Apparently there is a 
strong correlation between some parameters that leads to this singularity 
? Perhaps I misspecified the model for LMER (and LME) ?

Thank you very much for your help!
Best regards,

Francois