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st: RE: RE: RE: xtivreg, xtiveg2 and time invariant excluded instruments


From   "Schaffer, Mark E" <M.E.Schaffer@hw.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: RE: RE: xtivreg, xtiveg2 and time invariant excluded instruments
Date   Tue, 28 Sep 2010 18:00:21 +0100

Steve,

The behaviour of xtivreg is rather peculiar.  If you drop a variable by
hand, it seems reasonable that you should get the same results as when
the Stata command drops it automatically.  And since your original
estimation is unidentified, it also seems reasonable to expect xtivreg
to complain about it - I still don't understand what that first xtivreg
estimation actually is.

But at least you've answered your question!

--Mark

> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu 
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of sdm1
> Sent: 28 September 2010 17:44
> To: statalist@hsphsun2.harvard.edu
> Subject: st: RE: RE: xtivreg, xtiveg2 and time invariant 
> excluded instruments
> 
> Mark,
> 
> Thanks for pointing out the problem with time invariant 
> instruments in a panel FE model.
> 
> >>What happens if you reestimate using xtivreg and omit 
> lneedHPandG by 
> >>Hand.  Do you get the same results as reported below?
> 
> No, because now xtivreg automatically drops the endogenous 
> regressor (see below).  
> 
> xtivreg  lallcancersdsmr yr78dv yr89dv 
> (lg2_expphHPandG=llonepenhx lpoppucarx), fe
> 
> Fixed-effects (within) IV regression         Number of obs      =
> 456
> Group variable: pct_codenum                  Number of groups   =
> 152
> 
> R-sq:  within  = 0.2390                      Obs per group: min =
> 3
>        between =      .                                     avg =
> 3.0
>        overall = 0.0075                                     max =
> 3
> 
>                                              Wald chi2(2)       =
> 1.85e+07
> corr(u_i, Xb)  = -0.0000                     Prob > chi2        =
> 0.0000
> 
> --------------------------------------------------------------
> --------------
> --
> lallcance~mr |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
> Interval]
> -------------+------------------------------------------------
> ----------
> -------------+----
> --
> lg2_expphH~G |  (omitted)
>       yr78dv |  -.0141391   .0027198    -5.20   0.000    -.0194698
> -.0088084
>       yr89dv |  -.0264635   .0027198    -9.73   0.000    -.0317942
> -.0211328
>        _cons |   4.786809   .0019232  2489.00   0.000     4.783039
> 4.790578
> -------------+------------------------------------------------
> ----------
> -------------+----
> --
>      sigma_u |  .12299815
>      sigma_e |   .0237106
>          rho |  .96417042   (fraction of variance due to u_i)
> --------------------------------------------------------------
> --------------
> --
> F  test that all u_i=0:     F(151,302) =    62.74         
> Prob > F    =
> 0.0000
> --------------------------------------------------------------
> --------------
> --
> Instrumented:   lg2_expphHPandG
> Instruments:    yr78dv yr89dv llonepenhx lpoppucarx
> --------------------------------------------------------------
> --------------
> --
> 
> Steve
> 
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of 
> Schaffer, Mark E
> Sent: 27 September 2010 23:37
> To: statalist@hsphsun2.harvard.edu
> Subject: st: RE: xtivreg, xtiveg2 and time invariant excluded 
> instruments
> 
> Steve,
> 
> > -----Original Message-----
> > From: owner-statalist@hsphsun2.harvard.edu
> > [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of sdm1
> > Sent: 25 September 2010 16:55
> > To: statalist@hsphsun2.harvard.edu
> > Subject: st: xtivreg, xtiveg2 and time invariant excluded 
> instruments
> > 
> > I'm trying to estimate a panel fixed effects model (152 individuals 
> > observed annually over a 3 year period).  As you'll see 
> from the Stata 
> > 11 SE output below, xtivreg produces some estimates but 
> xtivreg2 seems 
> > to think that there are too few instruments.  I am puzzled 
> by this and 
> > why xtivreg drops the exogenous regressor lneedHPandG from 
> the model.  
> > Are either of these 'puzzles' related to the fact that both 
> excluded 
> > instruments (llonepenhx
> > lpoppucarx) are time invariant?
> 
> I think the answer to your question is "yes", but it still 
> leaves me puzzled.
> 
> Time-invariant variables turn into vectors of zeros after the 
> within-transformation, and drop out of the estimation.  That's why
> -xtivreg2- complains that the model is not identified.
> 
> What puzzles me is that xtivreg drops the variable 
> lneedHPandG instead of complaining about underidentification.
> 
> What happens if you reestimate using xtivreg and omit 
> lneedHPandG by hand, e.g.,
> 
> xtivreg  lallcancersdsmr yr78dv yr89dv 
> (lg2_expphHPandG=llonepenhx lpoppucarx), fe 
> 
> Do you get the same results as reported below?
> 
> --Mark
> 
> > 
> > Thanks.
> > 
> > Steve
> > 
> > . xtivreg  lallcancersdsmr lneedHPandG yr78dv yr89dv 
> > (lg2_expphHPandG=llonepenhx lpoppucarx), fe
> > 
> > Fixed-effects (within) IV regression         Number of obs      =
> > 456
> > Group variable: pct_codenum                  Number of groups   =
> > 152
> > 
> > R-sq:  within  =      .                      Obs per group: min =
> > 3
> >        between = 0.2229                                     avg =
> > 3.0
> >        overall = 0.1503                                     max =
> > 3
> > 
> >                                              Wald chi2(3)       =
> > 6.14e+06
> > corr(u_i, Xb)  = -0.6050                     Prob > chi2        =
> > 0.0000
> > 
> > --------------------------------------------------------------
> > --------------
> > --
> > lallcancer~r |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
> > Interval]
> > -------------+------------------------------------------------
> > ----------
> > -------------+----
> > --
> > lg2_expphH~G |  -.2661569   .1438617    -1.85   0.064    -.5481207
> > .0158069
> >  lneedHPandG |  (omitted)
> >       yr78dv |   .0135551   .0156954     0.86   0.388    -.0172073
> > .0443176
> >       yr89dv |   .0165872   .0237433     0.70   0.485    -.0299488
> > .0631232
> >        _cons |   4.116574   .3622872    11.36   0.000     3.406504
> > 4.826644
> > -------------+------------------------------------------------
> > ----------
> > -------------+----
> > --
> >      sigma_u |  .15213838
> >      sigma_e |  .04114184
> >          rho |  .93185438   (fraction of variance due to u_i)
> > --------------------------------------------------------------
> > --------------
> > --
> > F  test that all u_i=0:     F(151,301) =     9.87         
> > Prob > F    =
> > 0.0000
> > --------------------------------------------------------------
> > --------------
> > --
> > Instrumented:   lg2_expphHPandG
> > Instruments:    lneedHPandG yr78dv yr89dv llonepenhx lpoppucarx
> > --------------------------------------------------------------
> > --------------
> > --
> > 
> > . xtivreg2 lallcancersdsmr lneedHPandG yr78dv yr89dv 
> > (lg2_expphHPandG=llonepenhx lpoppucarx), fe equation not 
> identified; 
> > must have at least as many instruments not in the 
> regression as there 
> > are instrumented variables
> > 
> > *
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> > *   http://www.ats.ucla.edu/stat/stata/
> > 
> 
> 
> --
> Heriot-Watt University is a Scottish charity registered under 
> charity number SC000278.
> 
> 
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-- 
Heriot-Watt University is a Scottish charity
registered under charity number SC000278.


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