

   CCoonnssttaanntt VVaarriiaannccee FFuunnccttiioonn

        varIdent(value, form, fixed)

   AArrgguummeennttss::

      value: an optional numeric vector, or list of numeric
             values, with the variance function coefficients.
             If no grouping factor is present in `form', this
             argument is ignored, as the resulting variance
             function contains no coefficients. If `value' has
             length one, its value is repeated for all coeffi-
             cients in the variance function. If `value' has
             length greater than one, it must have length equal
             to the number of grouping levels minus one and
             names which identify its elements to the levels of
             the grouping factor. Only positive values are
             allowed for this argument. Default is
             `numeric(0)', which results in a vector of zeros
             of appropriate length being assigned to the coef-
             ficients when `object' is initialized (correspond-
             ing to constant variance equal to one).

       form: an optional one-sided formula of the form `~ v',
             or `~ v | g', specifying a variance covariate `v'
             and, optionally, a grouping factor `g' for the
             coefficients. The variance covariate is ignored in
             this variance function. When a grouping factor is
             present in `form', a different coefficient value
             is used for each of its levels less one reference
             level (see description section below). Defaults to
             `~ 1'.

      fixed: an optional numeric vector, or list of numeric
             values, specifying the values at which some or all
             of the  coefficients in the variance function
             should be fixed. It must have names identifying
             which coefficients are to be fixed. Coefficients
             included in `fixed' are not allowed to vary during
             the optimization of an objective function.
             Defaults to `NULL', corresponding to no fixed
             coefficients.

   DDeessccrriippttiioonn::

        This function is a constructor for the `varIdent'
        class, representing a constant variance function struc-
        ture. 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 vari-
        ance 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 vari-
        ance function. By default, if the elements in `value'
        are unnamed, the first group level is taken as the ref-
        erence level.

   VVaalluuee::

        a `varIdent' object representing a constant variance
        function structure, also inheriting from class `var-
        Func'.

   AAuutthhoorr((ss))::

        Jose Pinheiro and Douglas Bates

   SSeeee AAllssoo::

        `varWeights.varFunc', `coef.varIdent'

   EExxaammpplleess::

        library(lme)
        vf1 <- varIdent(c(F = 0.5), form = ~ 1 | Sex)

