strata: Identify Stratification Variables In survival: Survival Analysis

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

 `1` ```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.

`coxph`, `interaction`

Examples

 ```1 2 3 4 5 6``` ```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) ```

Example output

```[1] "1" "2" "3" "4"
[1] "low, 1"    "low, 2"    "low, 3"    "low, 4"    "medium, 1" "medium, 2"
[7] "medium, 3" "medium, 4" "high, 1"   "high, 2"   "high, 3"   "high, 4"
Call:
coxph(formula = Surv(futime, fustat) ~ age + strata(rx), data = ovarian)

coef exp(coef) se(coef)   z      p
age 0.1374    1.1472   0.0474 2.9 0.0038

Likelihood ratio test=12.7  on 1 df, p=0.000368
n= 26, number of events= 12
```

survival documentation built on Jan. 11, 2020, 9:38 a.m.