I'm doing stratified cox regression in R with the survival package:

```
cox <- coxph(response~strata(x), data=data)
```

This works fine, but when using the sampling package the strata function from the survival package is hidden and cannot be used directly.

However, calling it with the explicit namespace `survival::strata`

does not create a stratified analysis:

```
library(survival)
# sample data
mort28day <- c(1,0,1,0,0,0,0,1,0,0,0,1,1,0,1,0,0,1,0,1,1,1,1,0,1,1,1,0,0,1)
daysurv <- c(4,29,24,29,29,29,29,19,29,29,29,3,9,29,15,29,29,11,29,5,13,20,22,29,16,21,9,29,29,15)
levo <- c(0,0,0,0,1,1,1,0,0,0,0,0,0,0,0,0,0,1,0,0,1,0,0,0,0,0,0,0,0,0)
acbi30 <- data.frame(mort28day, daysurv, levo)
# create survival object
test.Surv <- Surv(acbi30$daysurv, acbi30$mort28day)
test.surv_fit_1 <- survfit(coxph(test.Surv~strata(levo), data=acbi30))
test.surv_fit_2 <- survfit(coxph(test.Surv~survival::strata(levo), data=acbi30))
```

So,

```
test.surv_fit_1$strata
# levo=0 levo=1
# 12 3
test.surv_fit_2$strata
# NULL
```

Why is this the case and how can I access this function when it is hidden by `sampling::strata`

(apart from detaching the sampling package ;-)

I'm using R version 2.15.2

Thanks, DeltaKappa

Update:

As a workaround to using survival::strata together with the sampling package I did

```
library(survival)
library(sampling, pos="package:base")
```

which loads the sampling package with a low search priority right above the base package. Then I can do

```
coxph(test.Surv~strata(levo), data=acbi30)
```

while calling `sampling::strata`

explicitly.

`coxph`

looks for certain functions, including`strata`

, in the supplied formula to identify as "special" terms. It calls`terms.formula`

(via`terms`

) with`special = c("strata", "cluster", "tt")`

, and it looks like this fails to identify`survival::strata`

as a special term. That missing special term then propagates throughout the rest of the calculations and leaves you with no strata.`terms.formula`

uses internal code, and I don't have the nerve (or perhaps the know-how) for source-surfing right now.`st <- strata(levo)`

and using it in the formula`test.Surv~st`

also didn't work... thanks for claryfing @BenBarnes