Where can I find out about using random effects models in R?
Paul Bleise has written an [attachment:nlmeR.pdf introductory guide ] to fitting random effects models in R using the nlme software with some case studies and general tips on using R objects.
The guide mentions, in particular, the function VarCorr() which computes residual variancesof models specified to VarCorr. Comparisons of VarCorrs() related to nested models enables the computation of percentage of variance explained by variable sets. For example if VarCorr(y~a+b) gives residual variance A and VarCorr(y~a) gives residual variance B (<=A) the proportion of variance explained by variable B = 1 - B/A.