If the following conditions are met in the QLIM procedure, the parameter estimates in the second stage equation of the Heckman's two-step sample selection model are incorrect:

• The HECKIT option is specified in the PROC QLIM statement.
• The SELECT option in the MODEL statement specifies a level a such that prob(z=a) = 1-F(x'β) in the first stage probit model.

For example, incorrect results are generated if the SELECT(z=0) option is specified where the levels of z are 0 or 1 in the first stage probit equation.

If an OUTPUT statement is specified, the statistics for the second stage equation and the inverse mills ratios for the first stage binary dependent variable in the output data set are also incorrect.

To circumvent the problem, create an alternative selection variable z1 such that z1=1 if z=0 and z1= 0 if z=1. Specify z1 as the dependent variable in the first stage probit equation, and specify SELECT(z1=1) in the second stage equation.