varClasses {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 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`

[Package nlme version 3.1-104 Index]