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st: interpreting heckprob

From   "Tomeka Davis" <[email protected]>
To   <[email protected]>
Subject   st: interpreting heckprob
Date   Sat, 16 Jan 2010 12:21:47 -0500

I have never used a Heckman selection model before and I have questions about interepreting the Stata output for a  Heckman probit model.  I have a dichotomous outcome and I am using a Heckman model to control for selection bias.  Specifically, my main dependent variable is course placement (honors or not) and I want to know if school type (private vs. public school) affects course placement.  Therefore, I am controlling for selection into private school.

I have run the model successfully, but it appears that the number of observations in the final model (N~5000) is greater than the number of cases that have complete data on the main dependent variable (N~3000).  Why is this the case? 

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