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st: treatreg with a continuous self-selected treatment


From   "David Parsley" <dparsley@owen.vanderbilt.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: treatreg with a continuous self-selected treatment
Date   Mon, 30 Jul 2007 14:09:52 -0500

Dear Statalist,

I am trying to model the effect of a treatment (x-variable) on an
outcome (y-variable), where the treatment is a self-selected, CONTINUOUS
variable (e.g., hours of study, dollars spent, etc.), and the outcome is
also continuous (performance).  I am interested in the effect of the
treatment on the outcome in the general population - not just the
treatment sample.  I observe y for the entire population (i.e.,
including those studying/spending, and those not).  My treatment is
either zero, or some positive value (lots of clustering at zero).

I have searched through econometrics texts/articles and (~) 99.99% of
them focus on self selection of the y-variable.  In my case, since the
selection is on the (continuous) x-variable, including the non-treatment
population would result in clustering at zero, and therefore biased
estimates. I have found discussions of treatment effects for dichotomous
independent variables, but not for 'simple' cases of continuous (though
strictly positive) treatments.

Treatreg seems to apply only to dichotomous varibles.

Thanks for any help.

David Parsley <http://mba.vanderbilt.edu/david.parsley/>
Owen Graduate School <http://mba.vanderbilt.edu/>
Vanderbilt University <http://mba.vanderbilt.edu/>
Nashville, TN 37203 <http://www.nashville.gov/flashpgs/flashhome.htm>
(615) 322-0649

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