Dear All
Are there any rules relating to the difference in the number of variables in
the selection equation and the structural equation when estimating the two
stage Heckman model?
The stata help files do suggest that the selection equation must have
atleast one variable that is not in the outcome equation - does this
necessarily mean that the number of variables in the selection equation are
always higher than in the outcome equation?
Also, how does one verify that this 'identifying' variable that seperates
the two equations is valid in the sense that it determines whether that case
is selected or not but does not determine the LHS in the second step?
Any insights would be much appreciated
Many thanks
Seema
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