This does not seem to be a case for -heckman-. -heckman- is for the
case when there is selection on the y variable in your case afqt. The
classic example is wage of women, where for some women we don't know
their wage because they don't work, and we expect that those women that
don't work would have earned less then average if they would have
worked. The reason that such selection on the y causes problems is
disccused graphically in a recent post:
http://www.stata.com/statalist/archive/2008-05/msg01176.html . Your use
of the -heckman- syntax, with a selection of YesPrep and NoPrep
suggests that that is not the case. In your case you should just use
-regress-:
regress afqt iv1 iv2 iv3 YesPrep
where the coefficient of YesPrep tells you how much better students do
when they prepare, while controlling for iv1 iv2 iv3.
Hope this helps,
Maarten
--- "Riemer, Richard A CIV DMDC" <[email protected]>
wrote:
> I want to use heckman on a program evaluation of the effectiveness of
> test preparation behavior on aptitude scores (afqt). Would I have to
> run heckman twice; once for test-prep (YesPrep) and once for
> non-test-prep (NoPrep) and then compare confidence intervals on the
> constant term (afqt _cons) to determine if there were a significant
> treatment effect on afqt scores? On the other hand, is there a
> direct
> way to test for a treatment effect, such as taking the output from
> heckman and running it through another procedure? Are there any good
> references for this question or examples in the literature where
> stata
> was used to test for program effectiveness.
>
> heckman afqt iv1 iv2 iv3, select(YesPrep=iv1 iv2 iv3)
> heckman afqt iv1 iv2 iv3, select(NoPrep =iv1 iv2 iv3)
>
-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands
visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434
+31 20 5986715
http://home.fsw.vu.nl/m.buis/
-----------------------------------------
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