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.
Value
x
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.