varClasses: Variance Function Classes
Description
Standard classes of variance function structures (varFunc)
available in the nlme library. Covariates included in the
variance function, denoted by variance covariates, may involve
functions of the fitted model object, such as the fitted values and
the residuals. Different coefficients may be assigned to the levels of
a classification factor.Value
Available standard classes ():varExpexponential of a variance covariate.varPowerpower of a variance covariate.varConstPowerconstant plus power of a variance covariate.varIdentconstant variance(s), generally used to allow
different variances according to the levels of a classification
factor.varFixedfixed weights, determined by a variance covariate.varCombcombination of variance functions.References
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models
in S and S-PLUS", Springer.