Standard classes of variance function structures (
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:
exponential of a variance covariate.
power of a variance covariate.
constant plus power of a variance covariate.
constant variance(s), generally used to allow different variances according to the levels of a classification factor.
fixed weights, determined by a variance covariate.
combination of variance functions.
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S and S-PLUS", Springer.