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. This style is now discouraged, use the cluster option instead.

### 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

coxph, survreg
marginal.model <- coxph(Surv(time, status) ~ rx, data= rats, cluster=litter,