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% $Id: getGroups.corStruct.Rd,v 1.6 2002/03/05 14:59:39 bates Exp $
\name{getGroups.corStruct}
\title{Extract corStruct Groups}
\usage{
\method{getGroups}{corStruct}(object, form, level, data, sep)
}
\alias{getGroups.corStruct}
\arguments{
\item{object}{an object inheriting from class \code{corStruct}
representing a correlation structure.}
\item{form}{this argument is included to make the method function
compatible with the generic. It will be assigned the value of
\code{formula(object)} and should not be modified.}
\item{level}{this argument is included to make the method function
compatible with the generic and is not used.}
\item{data}{an optional data frame in which to evaluate the variables
defined in \code{form}, in case \code{object} is not initialized and
the grouping factor needs to be evaluated.}
\item{sep}{character, the separator to use between group levels when
multiple levels are collapsed. The default is \code{'/'}.}
}
\description{
This method function extracts the grouping factor associated with
\code{object}, if any is present.
}
\value{
if a grouping factor is present in the correlation structure
represented by \code{object}, the function returns the corresponding
factor vector; else the function returns \code{NULL}.
}
\author{Jose Pinheiro \email{Jose.Pinheiro@pharma.novartis.com} and Douglas Bates \email{bates@stat.wisc.edu}}
\seealso{\code{\link{getGroups}}}
\examples{
cs1 <- corAR1(form = ~ 1 | Subject)
getGroups(cs1, data = Orthodont)
}
\keyword{models}
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