HCinitial               Two-Step Method for Parameter Estimation of the
                        Heckman Model
HeckmanBS               Heckman BS Model fit Function
HeckmanCL               Classic Heckman Model fit Function
HeckmanGe               Generalized Heckman Model Estimation
HeckmanSK               Normal Skew Model fit Function
HeckmantS               Heckman-t Model fit Function
IMR                     Inverse Mills Ratio
MEPS2001                Medical Expenditure Panel Survey
Mroz87                  U.S. Women's Labor Force Participation
PSID2                   Panel Study of Income Dynamics
RandHIE                 RAND Health Insurance Experiment
extract_model_components
                        Extract model components from formulas
nhanes                  US National Health and Nutrition Examination
                        Study
step2                   Heckman's two-step method
summary.HeckmanBS       Summary of Birnbaum-Saunders Heckman Model
summary.HeckmanCL       Summary of Classic Heckman Model
summary.HeckmanGe       Summary of Generalized Heckman Model
summary.HeckmanSK       Summary of Skew-Normal Heckman Model
summary.HeckmantS       Summary of Heckman-t Model
twostep                 Heckman's two-step method
