Create design matrix of sample selection models
# S3 method for selection
model.matrix(object, part = "outcome", ... )object of class selection.
character string indication which design matrix/matrices to extract: "outcome" for the design matrix/matrices of the outcome equation(s) or "selection" for the design matrix of the selection equation.
further arguments passed to other methods
(e.g. model.matrix.binaryChoice or model.matrix).
If argument part is "selection",
the design matrix of the selection equation is returned.
If argument part is "outcome",
the design matrix of the outcome equation (tobit-2 or treatment
model)
or a list of two outcome matrices (tobit-5 model) is returned.
All unobserved outcomes, including the corresponding explanatory
variables
are set to NA, even in case where
valid values were supplied
for estimation.
model.matrix, selection,
model.frame.selection, and selection-methods.