[R] lme between-group and within-group covariance

[MUHC-Research]

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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