R: Identify clusters.

cluster {survival} | R Documentation

## Identify clusters.

### Description

This is a special function used in the context of survival models. It identifies correlated groups of observations, and is used on the right hand side of a formula. Using `cluster()` in a formula implies that robust sandwich variance estimators are desired.

### Usage

    cluster(x)

### Arguments

`x` | A character, factor, or numeric variable.

### Details

The function's only action is semantic, to mark a variable as the cluster indicator. The resulting variance is what is known as the “working independence” variance in a GEE model. Note that one cannot use both a frailty term and a cluster term in the same model, the first is a mixed-effects approach to correlation and the second a GEE approach, and these don't mix.

### Value

`x`

### See Also

`coxph`, `survreg`

### Examples

    marginal.model <- coxph(Surv(time, status) ~ rx + cluster(litter), rats)
    frailty.model  <- coxph(Surv(time, status) ~ rx + frailty(litter), rats)

[Package survival version 2.37-7 Index]