There are a couple of things to notice. First, stata gave you estimates for two equations( The results of the selection equation are in the bottom and the results for the interest equation are on the top). Second, rho is the correlation between the errors in the two equations. stata gives you an estimate for rho, and tests that estimate. in your cas you can reject the null that rho=0, indeed you should be using a simple selection model on your data.
-------- Message d'origine--------
De: Katarina Lynch [mailto:[email protected]]
Date: jeu. 2004-11-25 13:09
À: [email protected]
Cc:
Objet: st: rho and wald test in heckman
In Heckman ML estimation how do I decide if there is a selection bias? On
the one hand, rho(the correlation between two equations) is non-zero, even
close to .95. On on the other hand, the Wald test at the bottom of the table
cannot reject the null hypothesis of independent equations. I have found a
power point presentation about heckman in STATA which sais that rho not
equal to zero is enough to infer that there is selection bias. If instead of
ML I do twostep, the IMR is insignificant, although there is nonzero
correlation between the outcome and selection equations. I am confused.
Would anyone spare a time to answer this simple question?
Thank you!
Katharina
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