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Re: st: atypical (?) 2sls question

From   "Gio Reidi" <>
Subject   Re: st: atypical (?) 2sls question
Date   Tue, 29 May 2007 21:32:21 +0100

Thanks a lot Austin,
indeed I found other treads speaking about the forbidden regression I
couldnt find earlier.
Thanks for having the patience to direct me to this problem.

On 5/29/07, Austin Nichols <> wrote:
Diego / Gio Reidi --
This is called the forbidden regression, and for good reason.  You
cannot plug in the fitted value. Instead, you have two endogenous
variables now, X and Ins*X, and you may need additional excluded
instruments.  You may want to consider interacting excluded
instruments... but be sure to check the diagnostic tests provided by
the newest copy of -ivreg2- (ssc install ivreg2, replace).

On 5/29/07, Gio Reidi <> wrote:
> Dear Statalisters,
> sorry if this will reach you twice..
> I hope you could give me an insight about this problem, concerning
> econometrical theory and Stata commands for 2sls.
> I am trying to run a sort of two stages least squares regressing the
> impact of some institutions on how variable Y responds to variable X
> (endogenous):
> Y=a+B(X)+Z(Institution*X)+K
> (exogenous variables, including main effect of the institution)
> To do so I want to instrument X, for which I have my good
> instrument(s). I do not have an instrument for the interactions
> between
>  X and the institution, but I was thinking (and  maybe I am completely
> wrong) that I would not need one if I just plug the fitted values of X
> from the first stage regression and construct with them the
> interaction term.  At the end, this is  what I want to assess: the
> impact on the "purified" X of the institutions. The goal is to obtain:
> Y=a+B(fittedX)+Z(Institution
> *fittedX)+K(exogenous variables).
> I cannot do this using ivreg or ivreg2 because then they would
> consider the interaction as an exogenous variable and insert it in the
> first stage. So I was thinking to do two stage least squares by
> hand,including all the exogenous regressors in the first stage plus
> the additional instruments. and then plugging in the fitted X series
> and the fitted X sinteracted with the institutions.
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