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RE: st: Tests of overidentifying restrictions with -ivregress-


From   DE SOUZA Eric <eric.de_souza@coleurope.eu>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Tests of overidentifying restrictions with -ivregress-
Date   Tue, 15 Oct 2013 17:04:15 +0200

That is why it is called a test of over-identifying restrictions and not a test of exogeneity. Though, you are right that many think it is a test of exogeneity.

Eric

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Santos Silva, Joao M C
Sent: 15 October 2013 16:43
To: statalist@hsphsun2.harvard.edu
Subject: RE: st: Tests of overidentifying restrictions with -ivregress-

Dear All,

I have been following this discussion with interest and I think people tend to read too much into the results of the overid test. Passing the test gives no information on whether the instruments identify the parameters of interest.
I guess this is now a well-known results, but we provide examples in this short paper:  http://bit.ly/18gLPeL

Hope this helps,

Joao

> Dear all,
> I need your help for interpreting some postestimation results of my 
> instrumental variables model. I am using Stata 12.0 and the command - 
> ivregress-. The sintax is the following:
> 
> ivregress 2sls dep (endo endoXexo = instrument1 instrument2 
> instrument1#exo
> instrument2#exo) exo exo1 exo2 exo3, first
> 
> where dep is the dependent variable, endo is the endogenous regressor, 
> exo is an exogenous regressor that I want to interact with the 
> endogenous one, and exo1, exo2, exo3 are other exogenous regressors.
> After running this model I type -estat overid- and I obtain this result:
> 
> 
> Tests of overidentifying restrictions:
> 
> Sargan (score) chi2(2) =  .311939  (p = 0.8556)
> Basmann chi2(2)        =  .310601  (p = 0.8562)
> 
> 
> This should mean that my instruments are not correlated with the error 
> of the main regression and therefore they are valid. Now, I want to 
> add an other exogenous regressor in the main regression, and for this reason I write:
> 
> ivregress 2sls dep (endo endoXexo = instrument1 instrument2 
> instrument1#exo
> instrument2#exo) exo exo1 exo2 exo3 exo4, first
> 
> where exo4 is the new variable that I add to the model. The effect of 
> this new factor on the dependent variable is statistically 
> significant, and it also considerably  reduces the effect of endo. 
> However, when I type again - estat overid-  the result is the following:
> 
> Tests of overidentifying restrictions:
> 
> Sargan (score) chi2(2) =  14.1205  (p = 0.0009)
> Basmann chi2(2)        =  14.0913  (p = 0.0009)
> 
> 
> This means that my instruments are not valid anymore. How it can be possible?
> The error term of the first model should incorporate also the effect of exo4.
> As far as I am aware, if my instruments are not correlated to it (the 
> error term), they can not be correlated with the error term of the 
> second model. I don't know how to interpret these results.....
> Any idea or suggestion?
> Thank you very much for help
> Roberto
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