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Re: st: Heckman selection model, collinearity problem


From   "Frederick J. Boehmke" <frederick-boehmke@uiowa.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Heckman selection model, collinearity problem
Date   Mon, 23 Jul 2007 10:21:31 -0400 (EDT)

If parttime is the indicator for whether an observation selects, then it only takes on the value 1 in the regression for the selected subsample. SO it can not be included. And it wouldn't really make sense even if it were.

Perhaps parttime is not dichotomous? Though Stata treats all values greater than zero as positive outcomes in the selection equation.

On Wed, 18 Jul 2007, Ying Zhou wrote:


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

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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