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st: st : how to treat endogeneous variables with NLSUR


From   orozco@toulouse.inra.fr
To   statalist@hsphsun2.harvard.edu
Subject   st: st : how to treat endogeneous variables with NLSUR
Date   Wed, 07 Nov 2007 14:28:01 +0100

Hi,

I already posted my question last week, but I didn't receive any ideas of
answer. Maybe I was unclear, so I try again to explain.

I just would like to know if it is possible to treat the problem of endogeneity
estimating a system of equations (more especially using NLSUR).
In fact, I suppose some variables to be endogeneous but I don't know how to
specify my equations of instrumentation (differently from my main system) and
test the validity of my variables (like with the overid command after ivreg2).

 I tried to write a little program estimating the following system of 2
 equations :
 w1 = a1 + a11 lnp1 + b1 lnp2 + b11 * (lnexp - lnP)
 w2 = a2 + a12 lnp2 + b2 lnp2^2 + b12 (lnexp - lnP)
 with lnP = a1 lnp1 + a2 lnp2 + a11*a12 + lnp1*lnp1 + lnp1*lnp2 + lnp2*lnp1 +
 lnp2*lnp2

 and I want to instrument the variable lnp2 that I suppose to be endogeneous
 (with 2 instruments : expfd and p1 for example).
 But it doesnt work and I have no idea how to correctly specify it.

 Thank you for trying to help me.

 valérie


 capture program drop nlsurtest

 program nlsurtest
     version 10
     syntax varlist(min=5 max=7) if, at(name)
     tokenize `varlist'
     args w1 w2 lnp1 lnp2 lnm expfd p1

     tempname a1 a2  a11 a12
     scalar `a1' = `at'[1,1]
     scalar `a2' = `at'[1,2]
     scalar `a11' = `at'[1,3]
     scalar `a12' = `at'[1,4]

     tempname b1 b2 b11 b12
     scalar `b1' = `at'[1,5]
     scalar `b2' = `at'[1,6]
     scalar `b11' = `at'[1,7]
     scalar `b12' = `b11'

     tempname t1 t2
     scalar `t1' = `at'[1,8]
     scalar `t2' = `at'[1,9]

     quietly {
         tempvar toto
         gen double `toto' = `a1' * `lnp1' + `a2' * `lnp2' + `a11' * `a12'
         forvalues i =  1/2 {
             forvalues j=1/2 {
                 replace `toto' = `toto' + `lnp`i'' * `lnp`j''
             }
         }
         replace `w1' = `a1' + `a11' * `lnp1' + `b1'*`lnp2' + `b11' * (`lnm' -
 `toto')
         replace `w2' = `a2' + `a12' * `lnp2' + `b2'*`lnp2'^2 + `b12' * (`lnm'
 -
 `toto')
         replace `lnp2' = `t1' * `expfd' + `t2' * `p1'  /*instrumentation
 equation*/

     }

 end

 use http://www.stata-press.com/data/r10/food,clear

 nlsur test  @ w1 w2 lnp1 lnp2 lnexp  expfd p1 ,parameters(a1 a2 a11 a12 b1 b2
 b11 ) neq(3) ifgnls





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