# varClasses

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##### Variance Function Classes

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.

Keywords
models
##### Value

• Available standard classes ():
• varExpexponential of a variance covariate.
• varPowerpower of a variance covariate.
• varConstPowerconstant plus power of a variance covariate.
• varIdentconstant variance(s), generally used to allow different variances according to the levels of a classification factor.
• varFixedfixed weights, determined by a variance covariate.
• varCombcombination 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.

varExp, varPower, varConstPower, varIdent, varFixed, varComb