st: predicted probabilities heckman and probit models

[Fabrizio Gilardi <Fabrizio.Gilardi@iepi.unil.ch>]

Hello,

I've run a heckman selection model with the two step procedure and 
asked Stata to compute predicted probabilities of selection:

heckman y1 x1, select(x2) twostep
predict pse1

Then I've run a probit with the same independent variables as in the 
selection stage of the heckman. The dependent variable is 1 if the 
outcome is observed in the heckman and 0 otherwise. Then I ask Stata 
to compute the predicted probabilities of a positive outcome:

probit y2 x2
predict p

The problem is that when I compare pse1 and p, it turns out that they 
are quite different. p>pse1 for 80 percent of the observations. I 
don't understand why.

Does this make any sense, or am I doing something wrong?

Many thanks in advance,

Fabrizio


--

Fabrizio Gilardi
Institut d'Etudes Politiques et Internationales
Universit� de Lausanne - BFSH2
1015 LAUSANNE
Switzerland

Bureau 5123
Phone: +41 21 692 31 48
Fax: +41 21 692 31 45
E-mail: fabrizio.gilardi@iepi.unil.ch

http://wwwpeople.unil.ch/fabrizio.gilardi/index.html

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