st: how to explain Rho in heckprob

["Jun Xu" <mystata@hotmail.com>]

Dear Listers:

I am using Stata's heckprob to estimate a heckit probit model. However, it 
seems there is little available out there re explaining Rho, for example, 
what's the meaning of Rho being positive or negative. I estimate four nested 
heckprob models, and as I add more variables in, the sign of Rho changes, 
and its significance level varies too. Any input?

Jun Xu
Department of Sociology
Indiana University at Bloomington

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