varClasses               package:nlme               R Documentation

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 (): 

  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.

Note:

     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'.

Author(s):

     Jose Pinheiro Jose.Pinheiro@pharma.novartis.com and Douglas Bates
     bates@stat.wisc.edu

References:

     Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S
     and S-PLUS", Springer.

See Also:

     'varComb', 'varConstPower', 'varExp', 'varFixed', 'varIdent',
     'varPower', 'summary.varFunc'

