This function is a constructor for the varIdent class,
representing a constant variance function structure. If no grouping
factor is present in form, the variance function is constant
and equal to one, and no coefficients required to represent it. When
form includes a grouping factor with $M > 1$ levels, the
variance function allows M different variances, one for each level of
the factor. For identifiability reasons, the coefficients of the
variance function represent the ratios between the variances and a
reference variance (corresponding to a reference group
level). Therefore, only $M-1$ coefficients are needed to represent
the variance function. By default, if the elements in value are
unnamed, the first group level is taken as the reference level.
Usage
varIdent(value, form, fixed)
Arguments
Value
a varIdent object representing a constant 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.