[R] Fixed variance structure for lme

[joanne ellis]

I have a data set with 50 different x values and 5 values for the sampling
variance; each of the 5 sampling variances  corresponds to 10 particular x
values. I am trying to fit a mixed effect linear model and I'm not sure
about the syntax for specifying the fixed variance structure. In Pinheiro's
book my situation appears to be similar to the example used for varIdent,
where there is a fixed value for variance for females, and a a different
fixed variance for males. The following syntax is used: varIdent(form~1|sex,
fixed=c(Female=.5)) or something to that effect. So for the 'fixed' part of
the argument, I need to specify that certain x'values have variance a,
certain ones have variance b, and so on. 
I assume that the formula argument in my case is written ~1|x which tells
the function that the variance is fixed and depends on the x-values.  Any
ideas for how to specify  different fixed variances for certain x-values?

Thanks
Joanne Ellis
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