

   IIddeennttiiffyy cclluusstteerrss

        cluster(x)

   AArrgguummeennttss::

          x: A character, factor, or numeric variable.

   DDeessccrriippttiioonn::

        This is a special function used in the context of the
        Cox model.  It identifies correlated groups of observa-
        tions, and is used on the right hand side of a formula.

   VVaalluuee::

        The function's only action is semantic, to mark a vari-
        able as the cluster indicator.

   SSeeee AAllssoo::

        `coxph'

   EExxaammpplleess::

        coxph(Surv(futime, fustat) ~ age + cluster(group))

