QLIM Procedure

(View the complete code for this example.)

In this example the data are generated such that the selection variable is discrete and the variable Y is truncated from below by zero. The program follows, with the results shown in Output 27.5.1:

title1 'Estimating a Sample Selection Model with Truncation';

data trunc;
   keep z y x1 x2;
   do i = 1 to 500;
      x1 = rannor( 19283 );
      x2 = rannor( 19283 );
      u1 = rannor( 19283 );
      u2 = rannor( 19283 );
      zl = 1 + 2 * x1 + 3 * x2 + u1;
      y = 3 + 4 * x1 - 2 * x2 + u1*.2 + u2;
      if ( zl > 0 ) then z = 1;
      else z = 0;
      if y>=0 then output;
   end;
run;

/*-- Sample Selection with Truncation --*/
proc qlim data=trunc method=qn;
   model z = x1 x2 / discrete;
   model y = x1 x2 / select(z=1) truncated(lb=0);
run;

In the following statements the data are generated such that the selection variable is discrete and the variable Y is censored from below by zero. The results are shown in Output 27.5.2.

title1 'Estimating a Sample Selection Model with Censoring';

data cens;
   keep z y x1 x2;
   do i = 1 to 500;
      x1 = rannor( 19283 );
      x2 = rannor( 19283 );
      u1 = rannor( 19283 );
      u2 = rannor( 19283 );
      zl = 1 + 2 * x1 + 3 * x2 + u1;
      yl = 3 + 4 * x1 - 2 * x2 + u1*.2 + u2;
      if ( zl > 0 ) then z = 1;
      else               z = 0;
      if ( yl > 0 ) then y = yl;
      else               y = 0;
      output;
   end;
run;

/*-- Sample Selection with Censoring --*/
proc qlim data=cens method=qn;
   model z = x1 x2 / discrete;
   model y = x1 x2 / select(z=1) censored(lb=0);
run;