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st: Heckman with covariates that perfectly predict selection


From   "K Ssdjk" <dfjkldf3@hotmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Heckman with covariates that perfectly predict selection
Date   Wed, 18 Dec 2002 12:22:54 -0500

I am running a heckman selection model (stata7, two-step estimation)
and have some RHS vars which perfectly predict censoring. While I get
estimates, the parameter on the perfect predictor seems to be the
opposite sign from what I would expect. For example I have an
indicator (Dmil) which is a perfect predictor: when Dmil=1 the
dependent variable is always observed. However the estimated
selection effect is negative,

...
select
Dmil -48.28 .
...

I understand the large value but why not positive? The rest of the
estimates are plausible and rho=-0.15. Any help/suggestions would be
appreciated!

KS

PS There might be a write-up in STB-43 about this but I could not
find it.

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