This is a special function used in the context of formula
used for Wilcoxon sum rank test for clustered data.
It identifies the cluster id of observations, and is used
on the right hand side of a formula.
Usage
cluster(x)
Arguments
x
A numeric variable of cluster id.
Value
x
Details
THe function's only action is semantic, to mark
a variable as the cluster indicator. If not supplied,
will assume no cluster in the data.