# Re: st: Heckman selection model, collinearity problem

 From "Zou Hong" To Subject Re: st: Heckman selection model, collinearity problem Date Tue, 24 Jul 2007 17:51:17 +0800

Dear Ying,

Maybe what you want is an instrumental approach rather than the pure sample selection.
In the first-stage, instrument parttime and in the second-stage use the fitted value of parttime in the wage regression
Joe

----- Original Message ----- From: "Frederick J. Boehmke" <frederick-boehmke@uiowa.edu>
To: <statalist@hsphsun2.harvard.edu>
Sent: Monday, July 23, 2007 10:21 PM
Subject: Re: st: Heckman selection model, collinearity problem

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>
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

*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/

```
- Fred

Note updated contact info (effective August 01, 2007):

< ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>  Frederick J. Boehmke
<  Associate Professor of Political Science
>  University of Iowa
<  341 Schaeffer Hall
>  Iowa City, IA 52242
<
>  Office: 361 Schaeffer
<  Phone:  319 335 2342
>  Fax:    319 335 3400
<
>  frederick-boehmke@uiowa.edu
<  http://www.fredboehmke.net
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/

```
```
*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/
```