[R] lme: howto specify covariance structure between levels of grouping factors

[Patrick Van Cleemputte]

Dear all,

I am studying the possibility of using the nlme package in R to analyse 
field trials of agricultural crops. I have a problem with the syntax for the 
modelling of variance covariance structures. I can model the within-group 
covariance structure using the correlation argument and the covariance 
structure between different random effects of the same grouping level using 
'random=pdDiag(~effect)|group' but I would like to model the covariance 
structure' between' the different levels of the grouping factor. This is 
necessary because the plants (= grouping factor) we are testing are not 
independant. They are genetically correlated and usually we know this 
correlation. I would therefore also like to specify the exact (not starting) 
values of this 'between subjects correlation'.

Can you tell me if this is possible in R and how the syntax of such a model 
would look like?

thank you in advance,

P.


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