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st: Heckman selection model, collinearity problem


From   "Ying Zhou" <ying.zhou@nuffield.ox.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Heckman selection model, collinearity problem
Date   Wed, 18 Jul 2007 12:47:32 +0100

Hi there
 
I am trying to do a Heckman selection model, looking at the effect of part-time contract on wages, while controlling for selection into part-time work. The syntax I use looks something like this 
heckman logwage educ age parttime, select (parttime = married children educ age)
 
But the output says that parttime is dropped (from the main equation) due to collinearity. I need to keep the part-time variable in the main equation in order to look at its effect on wages after controlling for selection into part-time work. Does anyone know how to solve this problem?
 
Thanks,
Ying
 
 

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