This function is a constructor for the varPower class,
representing a power variance function structure. Letting
$v$ denote the variance covariate and $\sigma^2(v)$
denote the variance function evaluated at $v$, the power
variance function is defined as $\sigma^2(v) = |v|^{2\theta}$, where $\theta$ is the variance
function coefficient. When a grouping factor is present, a different
$\theta$ is used for each factor level.
Usage
varPower(value, form, fixed)
Arguments
Value
a varPower object representing a power variance function
structure, also inheriting from class varFunc.
References
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models
in S and S-PLUS", Springer.