[R] random effects structure in lme
Vito Muggeo
vito.muggeo at giustizia.it
Thu Apr 22 15:27:27 CEST 2004
Dear all,
Some days ago I posted the same message below, but unfortunately with no
reply.
So, apologizes for this my re-sending, but I hope there is now someone
on-line that can help me....
In using the lme() function from the nlme package, I would like to specify a
particular correlation structure for the random effects.
For instance, for a 3 by 3 matrix, say, I am interested in assuming a
`quasi-diagonal' matrix having zero entries in all but the first column (and
the main
diagonal, of course)
Hence, given the three random effects b1 b2 and b3, I want to assume
cov(b2,b3)=0; given four random effects b1,b2,b3,b4 it should be
cov(b2,b3)=cov(b4,b2)=cov(b4,b3)=0.
For a simple diagonal matrix I specify `random=list(id=pdDiag(~1+x1+x2))',
(id is the grouping variable), but I am not able to go on....
Please, could anyone give me any advice?
many thanks,
vito
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