\Header{cluster}{Identify Clusters}
\keyword{survival}{cluster}
\begin{Description}\relax
This is a special function used in the context of the Cox model.  It
identifies correlated groups of observations, and is used on the right hand
side of a formula.
\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{Details}\relax
The function's only action is semantic, to mark a variable as the
cluster indicator.
\end{Details}
\begin{Value}
\code{x}
\end{Value}
\begin{SeeAlso}\relax
\code{\Link{coxph}}, \code{\Link{Surv}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}## Not run: 
coxph(Surv(futime, fustat) ~ age + cluster(group), data = ovarian)
## End(Not run)\end{ExampleCode}
\end{Examples}

