# strata: Identify Stratification Variables

## Description

This is a special function used in the context of the Cox survival model.
It identifies stratification variables when they appear on the right hand
side of a formula.

## Usage

strata(..., na.group=FALSE, shortlabel, sep=', ')

## Arguments

…

any number of variables. All must be the same length.

na.group

a logical variable, if `TRUE`

, then missing values are treated as a
distinct level of each variable.

shortlabel

if `TRUE`

omit variable names from resulting
factor labels. The default action is to omit the names if all of the
arguments are factors, and none of them was named.

sep

the character used to separate groups, in the created label

## Value

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

## Details

When used outside of a `coxph`

formula the result of the function
is essentially identical to the `interaction`

function,
though the labels from `strata`

are often more verbose.

## Examples

# NOT RUN {
a <- factor(rep(1:3,4), labels=c("low", "medium", "high"))
b <- factor(rep(1:4,3))
levels(strata(b))
levels(strata(a,b,shortlabel=TRUE))
coxph(Surv(futime, fustat) ~ age + strata(rx), data=ovarian)
# }