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st: -heckman-

From   Nishant Dass <>
To   STATA Help <>
Subject   st: -heckman-
Date   Mon, 16 Oct 2006 06:31:13 -0700 (PDT)

Dear list-members,

If I have the following Heckman selection model: 

y = a + b.X + e   (2nd stage)
z = c + d.W + u   (selection equation)

Would you know whether X and W have to be completely

I thought they ought to be distinct but I have seen some
papers that use variables that are common to both X and W. 

Does anyone have a good answer or know of a definitive
source on this?  

Thanks for your suggestions.


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