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.

`cluster(x)`

x

A character, factor, or numeric variable.

`x`

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.

# NOT RUN { marginal.model <- coxph(Surv(time, status) ~ rx, data= rats, cluster=litter, subset=(sex=='f')) frailty.model <- coxph(Surv(time, status) ~ rx + frailty(litter), rats, subset=(sex=='f')) # }