varFunc) structures available in the
nlme3 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.
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 the
varPower class.