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st: -corr()- problem in -reg3-


From   HJW <hjw48823@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: -corr()- problem in -reg3-
Date   Mon, 21 Feb 2005 01:21:06 -0800 (PST)

Dear listers,

I estimated a system of simultaneous equations using 3SLS via -reg3-
which, by default, assumes the equations are correlated. I then wish
to investigate the consequences if the correlations are restricted to
zero. I did this by adding -corr(independent)- option to -reg3-. To
my surprise, the resulting error covariance matrix is not diagonal as
it should be (advertised on p.308, Reference Manual [N-R]).

Am I missing something here? I know for my original purpose I can
apply 2SLS using -ivreg- to each of the equations, but I am wondering
why the -corr(independent)- does not work as advertised.

The following code demonstrates the issue.

====== demo begins =========

use http://www.stata-press.com/data/r8/supDem

 global demand "(qDemand: quantity price pcompete income)"
 global supply "(qSupply: quantity price praw)"

 reg3 $demand $supply, endog(price)
 mat list e(Sigma)

 reg3 $demand $supply, endog(price) corr(independent)
 mat list e(Sigma) /* should be diagonal */

==== demo ends =======

HJW

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