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st: RE: re: simultaneous equations


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: re: simultaneous equations
Date   Fri, 27 Feb 2009 15:42:46 -0000

With coefficients of the order of 1e8 there seems to be a massive
problem with the data. Look (again?) at the data, e.g. using a scatter
plot matrix. There could be a substantial outlier, or some built-in
dependency, or something else. Whatever it is, it is not obvious that
any model can make sense of the predictor set you are using. 

Nick 
n.j.cox@durham.ac.uk 

mmolina@uniroma3.it

As you adviced me I tried ivregress 2sls ... and obtained:

ivregress 2sls expshare b2b l1 age l4a (normscore = l11a l10),
note: l10 dropped because of collinearity

Instrumental variables (2SLS) regression               Number of obs =
198
                                                       Wald chi2(5)  =
40.30
                                                       Prob > chi2   =
0.0000
                                                       R-squared     =
  .
                                                       Root MSE      =
7.9e+07

------------------------------------------------------------------------
------
    expshare |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+----------------------------------------------------------
------
   normscore |   4.04e+08   3.93e+08     1.03   0.304    -3.66e+08
1.18e+09
         b2b |  -238087.7     387449    -0.61   0.539    -997473.9
521298.5
          l1 |   66040.49   14947.01     4.42   0.000     36744.89
95336.09
         age |   134492.7   220864.6     0.61   0.543    -298393.9
567379.3
         l4a |  -95654.77   20067.76    -4.77   0.000    -134986.9
-56322.68
       _cons |  -1.33e+08   1.18e+08    -1.13   0.260    -3.65e+08
9.85e+07
------------------------------------------------------------------------
------
Instrumented:  normscore
Instruments:   b2b l1 age l4a l11a

I agree with and don't know why the constant term is dropped.
I was looking for some causality link between normscore and expshare,
and
supossed Simultaneous Equations may help me.

Kit Baum <baum@bc.edu>

MMolina said

I run this command:


reg3 (normscore l1 b2b age l4a l11a l10) (expshare b2b l1 age normscore
l4a)

And obtained this results. Could you please tell me if the Std. Error

found for the expshare variable may invalidate the analysis or create
any
robustness problem?

I find it very strange, if expshare is really a fraction, that you get
coefficients on the order of 10^8 on any regressor. Would you try
running

ivreg expshare b2b l1 age l4a (normscore = l11a l10), first


and show us what you get from that regression? (you could also use
ivregress 2sls ... , first if you are using Stata 10)


Also, is there a good reason (given definitions of the variables) that
the
constant term is dropped in your first equation?


This system is triangular, in that normscore can be estimated with OLS
as
it does not contain expshare. You may still want to estimate it with
3SLS,
but something fishy is going on here. See what the single- equation
estimates of your second equation reveal.

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