Variance Function Classes
Standard classes of variance function structures (
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.
- Available standard classes ():
varExp exponential of a variance covariate. varPower power of a variance covariate. varConstPower constant plus power 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.
Users may define their own
varFunc classes by specifying a
constructor function and, at a minimum, methods for the
examples of these functions, see the methods for class