

   IIddeennttiiffyy ssttrraattaa vvaarriiaabblleess..

        strata(..., na.group=F)

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

        ...: Any number of variables.  All must be the same
             length.

   na.group: if set to T, then missing values are treated as a
             distinct level of each variable.

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

        The result is identical to the interaction() function,
        but for the labeling of the factors (strata is more
        verbose).

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

        a new factor, whose levels are all possible combina-
        tions of the factors supplied as arguments.

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

        `coxph'

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

        coxph(Surv(futime, fustat) ~ age + strata(rx))

