The right hand side of
form, or of each element of
form,
when given as a list, is evaluated in
data and must result in a
factor, or an integer vector. In the latter case, the unique elements
of the stratification variable, when sorted, must form a sequence of
consecutive integers.
USAGE:
getStrata(object, form)
REQUIRED ARGUMENTS:
object
an object inheriting from class
data.frame.
form
a one-sided formula specifying an stratification
variable, or a list of one-sided formulas specifying stratification
variables.
VALUE:
either an integer vector with values ranging from zero to the
number of strata minus one, or, when
form is given as a list, a
list of such integer vectors.