[R-sig-ME] Different within-group error correlation matrices per group

[Luis Damiano]

Dear all,

I am working with lme and I would like to have different within-group error
correlation matrices per group (\Lambda_i in 5.1 from Pinheiro & Bates).

Currently, my sentence looks like the following

correlation = corCompSymm(form = ~ 1 | ind)


which imposes the SC structure into the within-group error correlation
matrix, marking the individuals by "ind".

According to the following example in SAS given by an instructor, it is
possible to estimate different within-group error correlation matrices per
sex using the following sentences (see "group=gender" in fourth line):

proc mixed  data=dental;
>   class  child gender;
>   model distance = gender gender*age / noint solution;
>   repeated / group=gender subject=child;
>   random intercept age / type=un subject=child g gcorr v vcorr;
> run;


I cannot figure out how to reproduce this in R. I took a look at the
documentation as well as the first five chapters of Pinheiro & Bates (2000)
without luck.

Cheers!

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