Create design matrix of endogenous treatment model
# S3 method for etregrob
model.matrix(object, part = "outcome", ...)object of class etregrob
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.