Variance Function Classes
Standard classes of variance function structures (varFunc) available in the nlme package. 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.
Available standard classes: - varExp: exponential of a variance covariate.
varPower: power of a variance covariate.
varConstPower: constant plus power of a variance covariate.
varConstProp: constant plus proportion of a variance covariate.
varIdent: constant variance(s), generally used to allow different variances according to the levels of a classification factor.
varFixed: fixed weights, determined by a variance covariate.
varComb: combination of variance functions.
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S and S-PLUS", Springer.
José Pinheiro and Douglas Bates bates@stat.wisc.edu
Users may define their own varFunc classes by specifying a constructor function and, at a minimum, methods for the functions coef, coef<-, and Initialize. For examples of these functions, see the methods for class varPower.
varComb, varConstPower, varConstProp, varExp, varFixed, varIdent, varPower, summary.varFunc