# varClasses

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##### Variance Function Classes

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.

Keywords
models
##### 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.

##### References

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

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