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.

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.

See also

coxph, interaction

Examples

a <- factor(rep(1:3,4), labels=c("low", "medium", "high"))
b <- factor(rep(1:4,3))
levels(strata(b))
#> [1] "1" "2" "3" "4"
levels(strata(a,b,shortlabel=TRUE))
#>  [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"  

coxph(Surv(futime, fustat) ~ age + strata(rx), data=ovarian) 
#> Call:
#> coxph(formula = Surv(futime, fustat) ~ age + strata(rx), data = ovarian)
#> 
#>        coef exp(coef) se(coef)     z       p
#> age 0.13735   1.14723  0.04741 2.897 0.00376
#> 
#> Likelihood ratio test=12.69  on 1 df, p=0.0003678
#> n= 26, number of events= 12