kmanSK, HeckmanBS, and HeckmantS.
Additionally, it can handle covariate matrices for modeling dispersion (sigma) and
correlation (rho) structures."><meta property="og:description" content="This internal utility function extracts key components—such as model frames, matrices,
and response variables—from formulas and a data set. It is used by models like
HeckmanCL, HeckmanGe, HeckmanSK, HeckmanBS, and HeckmantS.
Additionally, it can handle covariate matrices for modeling dispersion (sigma) and
correlation