\HeaderA{varPower}{Power Variance Function}{varPower}
\keyword{models}{varPower}
\begin{Description}\relax
This function is a constructor for the \code{varPower} class,
representing a power variance function structure. Letting
\eqn{v}{} denote the variance covariate and \eqn{\sigma^2(v)}{s2(v)}
denote the variance function evaluated at \eqn{v}{}, the power
variance function is defined as \eqn{\sigma^2(v) = |v|^{2\theta}}{s2(v) =
|v|^(2*t)}, where \eqn{\theta}{t} is the variance 
function coefficient. When a grouping factor is present, a different
\eqn{\theta}{t} is used for each factor level.
\end{Description}
\begin{Usage}
\begin{verbatim}
varPower(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. \code{Value} must have
length one, unless a grouping factor is specified in \code{form}.
If \code{value} has length greater than one, it must have names
which identify its elements to the levels of the grouping factor
defined in \code{form}. If a grouping factor is present in
\code{form} and \code{value} has length one, its value will be
assigned to all grouping levels. 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 must evaluate to a numeric vector and may involve
expressions using \code{"."}, representing  a fitted model object
from which fitted values (\code{fitted(.)}) and residuals
(\code{resid(.)}) can be extracted (this allows the variance
covariate to be updated during the optimization of an object
function). When a grouping factor is present in \code{form},
a different coefficient value is used for each of its
levels. 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{} fitted(.)}
representing a variance covariate given by the fitted values of a
fitted model object and no grouping factor. 
\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. If a grouping factor is
specified in \code{form}, \code{fixed} 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{varPower} object representing a power 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{\LinkA{varWeights.varFunc}{varWeights.varFunc}}, \code{\LinkA{coef.varPower}{coef.varPower}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
vf1 <- varPower(0.2, form = ~age|Sex)
\end{ExampleCode}
\end{Examples}

