varFixed                package:nlme                R Documentation

Fixed Variance Function

Description:

     This function is a constructor for the 'varFixed' class,
     representing a variance function with fixed variances. Letting v
     denote the variance covariate defined in 'value', the variance
     function s2(v) for this class is s2(v)=|v|. The variance covariate
     v is evaluated once at initialization and remains fixed
     thereafter. No coefficients are required to represent this
     variance function.

Usage:

     varFixed(value)

Arguments:

   value: a one-sided formula of the form '~ v' specifying a variance
          covariate 'v'. Grouping factors are ignored.

Value:

     a 'varFixed' object representing a fixed variance function
     structure, also inheriting from class 'varFunc'.

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:

     'varClasses', 'varWeights.varFunc', 'varFunc'

Examples:

     vf1 <- varFixed(~age)

