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st: Some questions about ivreg2 xtivreg xtivreg2

From   "" <>
Subject   st: Some questions about ivreg2 xtivreg xtivreg2
Date   Tue, 13 Nov 2007 18:10:19 +0000

Dear users, I'm a newbie and not a great econometrician...

I'm trying to estimate a linear equation model (unbalanced panel database) where one regressor is endogenous:

* yit = x1it x2it x3it uit

yit is the dependent variable
x1it and x2it are exogenous
x3it is endogenous
uit error

By Stata, I run the following model:

sort id year
tsset id year

* ivreg2 y1 x1 x2 (x3 = x4 x5 x6), first bw(2) gmm2s kernel(tru) robust

(note: x4 and x5 are suggested by economic theory)

Then, I was suggested to compute the same equation by "xtivreg2", getting very different result (and the insignificance of one instrument)! 
I know that as estimation methods differ you have something of a check on whether models
are stable, efficient and so on, but I really don't understand why and how it would be better estimating the equation above by xtivreg2 (or xtvreg). Moreover: 

1) what are the differences and the reason of choice between: ivreg2 (WITH gmm option) and xtivreg and xtvreg2?  
2) What does concretely mean that xtvreg2 is a "wrapper" for ivreg2 (from the help of Stata)?
3) Would be incomplete an econometric analysis employing an ivreg2 estimation only?
4) For xtivreg2, the instruments for the endogenous variable could be the lagged values of the endogenous or are usually the same of a previous ivreg2?
5) More in general are there papers or articles able to explain the reason for one choice of computation or for the other? These are new methods and I'm a bit confused.

Many thanks for any answer to the above questions and for any suggestion regarding the steps I should follow in my econometric research.


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