Standard classes of variance function structures (`varFunc`

)
available in the `nlme`

package. 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.

Available standard classes:

- varExp
exponential of a variance covariate.

- varPower
power of a variance covariate.

- varConstPower
constant plus power of a variance covariate.

- varConstProp
constant plus proportion 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.

José Pinheiro and Douglas Bates bates@stat.wisc.edu

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

`varComb`

,
`varConstPower`

,
`varConstProp`

,
`varExp`

,
`varFixed`

,
`varIdent`

,
`varPower`

,
`summary.varFunc`