# strata

##### Identify Stratification Variables

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.

- Keywords
- survival

##### 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

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

```
# 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)
# }
```

*Documentation reproduced from package survival, version 3.1-7, License: LGPL (>= 2)*