strata {survival} | R Documentation

## 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

### Details

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

### Value

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

### See Also

`coxph`, `interaction`

### Examples

    a <- factor(rep(1:3,4), labels="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)

[Package survival version 2.39-4 Index]