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st: 1st stage coefficients when using cmp


From   v nguyen <katiana00@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: 1st stage coefficients when using cmp
Date   Thu, 1 Oct 2009 12:45:16 -0700 (PDT)

Hello,

I've been using the cmp command to estimate a 2-equation system in which one of the variables is endogenous. To be concrete, my estimation looks thusly:

--------------------------
svy: cmp 

(D12: MRJMON = UNEMPL UNEMR_NSA MRJPRICE COCPRICE HERPRICE METHPRICE 
RACE MAR IRAGE IRAGE2 IREDUC NCHILD CITY RURORURB NONWAGE) 

(E12: UNEMPL=H_MEAN UNEMR_NSA MRJPRICE COCPRICE HERPRICE METHPRICE  RACE MAR IRAGE IRAGE2 IREDUC NCHILD CITY RURORURB NONWAGE), 

ind($cmp_probit $cmp_probit)
--------------------------

So, in my model, UNEMPL (unemployment) is endogenous and is instrumented for by H_MEAN. My question is why is it that the "first-stage" coefficients do not stay the same when I only change the dependent variable in the "second-stage"? For example, if I change the above equation to

--------------------------
svy: cmp 

(D13: MRJMONF = UNEMPL UNEMR_NSA MRJPRICE COCPRICE HERPRICE METHPRICE  RACE MAR IRAGE IRAGE2 IREDUC NCHILD CITY RURORURB NONWAGE) 

(E13: UNEMPL=H_MEAN UNEMR_NSA MRJPRICE COCPRICE HERPRICE METHPRICE  RACE MAR IRAGE IRAGE2 IREDUC NCHILD CITY RURORURB NONWAGE), 

ind($cmp_probit $cmp_probit)
--------------------------

That is, I only changed MRJMON to MRJMONF in the D13 equation, but I get that the coefficients for the E13 equation, e.g. the coefficients on H_MEAN UNEMR_NSA . . . etc. are not the same as the coefficients in the E12 equation. Why is that?

Thank you for any help you can offer,

Vuong



      
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