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 |

`shortlabel` |
if |

`sep` |
the character used to separate groups, in the created label |

### 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.

### Value

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

### See Also

### Examples

```
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)
```

*survival*version 3.7-0 Index]