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