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Re: st: re: ivreg2 running OLS not 2SLS

From   "Austin Nichols" <>
Subject   Re: st: re: ivreg2 running OLS not 2SLS
Date   Thu, 20 Sep 2007 11:23:50 -0400

If all your excluded instruments are dropped in the "first-stage"
regression, the IV estimate is the OLS estimate. If Z is collinear
with X, you do not have excluded instruments!

qui tab educ, gen(ded)
ivreg2 lwage exper expersq (educ=ded*)
reg lwage exper expersq educ

On 9/20/07, Kit Baum <> wrote:
> Nirina said
> I run ivreg2 and it is telling me that my endogenous variable is
> collinear with the instruments and then it is running OLS but still
> reporting Sargan test of overid.
> I find it wierd since this is what I would have expected it to do:
> drop the collinear variables from the list of  instruments and then
> still run 2SLS. Could anyone please help on what could be the trick?
> Per the Statalist FAQ, Nirina should show us her commands and the
> resulting output. Otherwise it is hard to conjecture what might cause
> the reported behavior.
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