nlme (version 3.1-99)

varIdent: Constant Variance Function

Description

This function is a constructor for the varIdent class, representing a constant variance function structure. If no grouping factor is present in form, the variance function is constant and equal to one, and no coefficients required to represent it. When form includes a grouping factor with $M > 1$ levels, the variance function allows M different variances, one for each level of the factor. For identifiability reasons, the coefficients of the variance function represent the ratios between the variances and a reference variance (corresponding to a reference group level). Therefore, only $M-1$ coefficients are needed to represent the variance function. By default, if the elements in value are unnamed, the first group level is taken as the reference level.

Usage

varIdent(value, form, fixed)

Arguments

value

an optional numeric vector, or list of numeric values, with the variance function coefficients. If no grouping factor is present in form, this argument is ignored, as the resulting variance function contains no coefficients. If

form

an optional one-sided formula of the form ~ v, or ~ v | g, specifying a variance covariate v and, optionally, a grouping factor g for the coefficients. The variance covariate is ignored in th

fixed

an optional numeric vector, or list of numeric values, specifying the values at which some or all of the coefficients in the variance function should be fixed. It must have names identifying which coefficients are to be fixed. Coefficients

Value

• a varIdent object representing a constant variance function structure, also inheriting from class varFunc.

References

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

See Also

varClasses, varWeights.varFunc, coef.varIdent

Examples

vf1 <- varIdent(c(Female = 0.5), form = ~ 1 | Sex)