 Usage:  heckit = heckman(x,y,z,q)  
 
 Input:

  x                      n x d matrix , the observed explanatory variables 
                         of the regression equation 
                         
  y                      n x 1 matrix , the observed response variable of the 
                         regression equation 
                         
  z                      n x p matrix , the observed explanatory variables 
                         of the selection equation 
                         
  q                      n x 1 matrix , the observed response variable of the 
                         regression equation; 
                         
 Output:

  heckit.b               d x 1 vector, contains the estimated coefficients of the components of x 
                         result of the second step 
                         
  heckit.s               scalar, contains the estimated covariance of the error terms 
                         in the selection equation and regression equation 
                         result of the second step 
                         
  heckit.g               p x 1 vector, contains the estimated coefficients of the components of z, 
                         result of the first step 
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
