
Create design matrix of sample selection model
# S3 method for heckitrob
model.matrix(object, part = "outcome", ...)
object of class heckitrob
character string indicating which matrix to extract: "outcome" for the outcome equation, "selection" for the selection equation.
currently not used
If argument part
is "outcome"
, the design matrix of the outcome equation is returned.
If argument part
is "selection"
, the design matrix of the selection equation is returned.