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st: Heckman comand sample issue


From   "Nicolas Theopold" <Nicolas.Theopold@economics.oxford.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Heckman comand sample issue
Date   Tue, 20 Apr 2004 10:45:53 +0100

Dear Statalist users,

please excuse if this is a basic question, however, I could not find the
answer or the right search terms to find it in the archives.
I have a data set with about 600,000 observations, of which about 90,000
are wage earners. In it, I would like to run the heckman command to
correct for sample selectivity bias. My earnings variable is ln(wages),
and thus has missing values for all observations that do not have a
recorded wage. Hence, if I run my heckman with:

Heckman ln_wages varlist1, sel(ilf = varlist1 varlist2), 

where varlist2 are my exclusion restrictions and ilf is a participation
dummy, the number of observations comes down to 90,000 (ilf does not
only take the value of 1, since I failed to clean the data completely,
yet).
My aim is to run the selection function on the whole sample (of 600,000
obs), and to get stata to run the wage function without the missing
observations. The only solution I found was to replace all missing
values with -9999 in ln_wages.

Is there some better way to tell stata to use the whole sample for the
selection function, even if ln_wages is missing?

Thank you very much in advance for your help.

Nicolas

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