\Header{varIdent}{Constant Variance Function}
\keyword{models}{varIdent}
\begin{Description}\relax
This function is a constructor for the \code{varIdent} class,
representing a constant variance function structure. If no grouping
factor is present in \code{form}, the variance function is constant
and equal to one, and no coefficients required to represent it. When
\code{form} includes a grouping factor with \eqn{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 \eqn{M-1}{} coefficients are needed to represent
the variance function. By default, if the elements in \code{value} are
unnamed, the first group level is taken as the reference level.
\end{Description}
\begin{Usage}
\begin{verbatim}
varIdent(value, form, fixed)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{value}] an optional numeric vector, or list of numeric values,
with the variance function coefficients. If no grouping factor is
present in \code{form}, this argument is ignored, as the resulting
variance function contains no coefficients. If \code{value} has
length one, its value is repeated for all coefficients in the
variance function. If \code{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 \code{numeric(0)}, which results in a vector of zeros of
appropriate length being assigned to the coefficients when
\code{object} is initialized (corresponding to constant variance 
equal to one).
\item[\code{form}] an optional one-sided formula of the form \code{\textasciitilde{} v}, or
\code{\textasciitilde{} v | g}, specifying a variance covariate \code{v} and,
optionally, a grouping factor \code{g} for the coefficients. The
variance covariate is ignored in this variance function. When a
grouping factor is present in \code{form}, a different coefficient
value is used for each of its levels less one reference level (see
description section below). Several grouping variables may be
simultaneously specified, separated by the \code{*} operator, like
in \code{\textasciitilde{} v | g1 * g2 * g3}. In this case, the levels of each
grouping variable are pasted together and the resulting factor is
used to group the observations. Defaults to \code{\textasciitilde{} 1}. 
\item[\code{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 \code{fixed} are not allowed to vary during the
optimization of an objective function. Defaults to \code{NULL},
corresponding to no fixed coefficients.
\end{ldescription}
\end{Arguments}
\begin{Value}
a \code{varIdent} object representing a constant variance function
structure, also inheriting from class \code{varFunc}.
\end{Value}
\begin{Author}\relax
Jose Pinheiro \email{Jose.Pinheiro@pharma.novartis.com} and Douglas Bates \email{bates@stat.wisc.edu}
\end{Author}
\begin{SeeAlso}\relax
\code{\Link{varWeights.varFunc}}, \code{\Link{coef.varIdent}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
vf1 <- varIdent(c(Female = 0.5), form = ~ 1 | Sex)
\end{ExampleCode}
\end{Examples}

