\HeaderA{cluster}{Identify Clusters}{cluster}
\keyword{survival}{cluster}
\begin{Description}\relax
This is a special function used in the context of survival models.  It
identifies correlated groups of observations, and is used on the right hand
side of a formula.  Using \code{cluster()} in a formula implies that
robust sandwich variance estimators are desired.
\end{Description}
\begin{Usage}
\begin{verbatim}
cluster(x)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x}] A character, factor, or numeric variable.

\end{ldescription}
\end{Arguments}
\begin{Value}
\code{x}
\end{Value}
\begin{SeeAlso}\relax
\code{\LinkA{coxph}{coxph}}, \code{\LinkA{Surv}{Surv}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
data(kidney)
frailty.model <- coxph(Surv(time, status)~ age + sex + disease + frailty(id), kidney)
marginal.model <- coxph(Surv(time, status)~ age + sex + disease+cluster(id), kidney)
summary(frailty.model)
summary(marginal.model)

data(rats)
frailty.model <- survreg(Surv(time, status) ~ rx +  frailty(litter), rats )
marginal.model <- survreg(Surv(time, status) ~ rx +  cluster(litter), rats )
summary(frailty.model)
summary(marginal.model)

\end{ExampleCode}
\end{Examples}