

   VVaarriiaannccee FFuunnccttiioonn CCllaasssseess

   DDeessccrriippttiioonn::

        Standard classes of variance function structures (`var-
        Func') available in the `lme' 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.

   VVaalluuee::

        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 dif-
             ferent variances according to the levels of a
             classification factor.

   varFixed: fixed weights, determined by a variance covariate.

    varComb: combination of variance functions.

   NNoottee::

        Users may define their own `varFunc' classes by speci-
        fying a `constructor' function and, at a minimum, meth-
        ods for the functions `coef', `coef<-', and `initial-
        ize'. For examples of these functions, see the methods
        for class `varPower'.

   AAuutthhoorr((ss))::

        Jose Pinheiro and Douglas Bates

   SSeeee AAllssoo::

        `varExp', `varPower', `varConstPower', `varIdent',
        `varFixed', `varComb'

