lme between-group and within-group covariance
I will try to make this more precise. In the lme() function, the "correlation" argument allows the user to specify a within-group correlation structure, i.e. the structure of the Lambda matrix using the mixed model notation in Pineiro and Bates. What I want to do is specify a distinct structure for the Psi matrix (same notation), that is, a correlation structure for the random effects. If lme() doesn't allow for this, is there any other function that I could use?
MUHC-Research wrote:
Dear R users, I would be interested in using the lme() function to fit a linear mixed model to a longitudinal dataset. I know this function allows for the specification of a within-group covariance structure. However, does it allow for the explicit specification of a between-group covariance structure? Being able to specify both separately would be very important in the context of my project since, as might be expected, they have different implications/interpretations. For instance, the mixed procedure in SAS allows users to specify the two structures separately by adding a value for the type argument after the RANDOM statement and the REPEATED statement. My question is thus if we can do the same with lme(). I thank you most sincerely for your help.
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