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Re: st: RE: Blown up IV coefficient


From   Shikha Sinha <[email protected]>
To   [email protected]
Subject   Re: st: RE: Blown up IV coefficient
Date   Wed, 9 Mar 2011 13:25:00 -0500

Jeff, thanks.

Should the reduced-form also include the dependent variable
(agr_share) as mentioned by you?
 areg roaddum rank  ims_hab_pop, absorb(dcode) robust

Sign on rank is expected, but it is statistically insignificant...

Thanks,
Shikha

On Wed, Mar 9, 2011 at 12:24 PM, Wooldridge, Jeffrey <[email protected]> wrote:
> For the IV estimates to be at all convincing, first run the reduced
> form,
>
> areg roaddum rank agr_share  ims_hab_pop, absorb(dcode) robust
>
> and look at the sign of the coefficient on rank and its t statistic.
> Hopefully you have a predicted sign for the effect of rank on roaddum.
> If the sign is what you expect, is rank strongly statistically
> significant? The statistics for weak identification provide this
> information, too, but you should estimate the reduced form, anyway. It
> is easier to see what is happening.
>
> My guess is the instrument is weak. Or, the IV could actually be
> endogenous (which cannot be tested, unfortunately).
>
> Jeff
>
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of Shikha Sinha
> Sent: Wednesday, March 09, 2011 12:17 PM
> To: [email protected]
> Subject: st: Blown up IV coefficient
>
> Dear all,
>
> I am estimating an instrumental variable model by using -ivreg2 in
> Stata. The IV estimates are way more than OLS estimate (4.006567).
> Could you advise what could be possible reasons for such a blown up IV
> coeff (49.97424 )? or my interpretation of coeff is incorrect.
>
>  areg agr_share roaddum ims_hab_pop, robust absorb(dcode)
>
> Linear regression, absorbing indicators                Number of obs =
> 266
>                                                       F(  2,   250) =
> 0.66
>                                                       Prob > F      =
> 0.5182
>                                                       R-squared     =
> 0.0680
>                                                       Adj R-squared =
> 0.0121
>                                                       Root MSE      =
> 26.294
>
> ------------------------------------------------------------------------
> ------
>             |               Robust
>   agr_share |      Coef.   Std. Err.      t    P>|t|     [95% Conf.
> Interval]
> -------------+----------------------------------------------------------
> ------
>     roaddum |   4.006567   3.572553     1.12   0.263     -3.02957
> 11.0427
>  ims_hab_pop |    .001926   .0344155     0.06   0.955    -.0658552
> .0697071
>       _cons |   34.78717   36.38659     0.96   0.340    -36.87615
> 106.4505
> -------------+----------------------------------------------------------
> ------
>       dcode |   absorbed                                      (14
> categories)
>
> . xi: ivreg2 agr_share  ims_hab_pop i.dcode (roaddum=rank), robust
> i.dcode           _Idcode_1-14        (_Idcode_1 for dcode==UP09
> omitted)
>
> IV (2SLS) estimation
> --------------------
>
> Estimates efficient for homoskedasticity only
> Statistics robust to heteroskedasticity
>
>                                                      Number of obs =
> 266
>                                                      F( 15,   250) =
> 0.88
>                                                      Prob > F      =
> 0.5891
> Total (centered) SS     =  185457.6241                Centered R2   =
> -0.5185
> Total (uncentered) SS   =       590824                Uncentered R2 =
> 0.5233
> Residual SS             =   281625.775                Root MSE      =
> 32.54
>
> ------------------------------------------------------------------------
> ------
>             |               Robust
>   agr_share |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
> Interval]
> -------------+----------------------------------------------------------
> ------
>     roaddum |   49.97424   35.09856     1.42   0.154    -18.81768
> 118.7662
>  ims_hab_pop |  -.0354515   .0511597    -0.69   0.488    -.1357226
> .0648196
>   _Idcode_2 |   -39.0505   25.58464    -1.53   0.127    -89.19546
> 11.09446
>   _Idcode_3 |  -41.62481   26.03165    -1.60   0.110    -92.64591
> 9.396298
>   _Idcode_4 |  -16.54727   20.19038    -0.82   0.412    -56.11968
> 23.02513
>   _Idcode_5 |  -41.45807   29.51111    -1.40   0.160    -99.29878
> 16.38264
>   _Idcode_6 |  -32.93124   16.32568    -2.02   0.044      -64.929
> -.9334909
>   _Idcode_7 |  -8.880318   13.20529    -0.67   0.501    -34.76221
> 17.00157
>   _Idcode_8 |  -44.38007   24.23395    -1.83   0.067    -91.87774
> 3.117595
>   _Idcode_9 |  -44.81575   24.87717    -1.80   0.072    -93.57409
> 3.942605
>  _Idcode_10 |  -21.14303   12.84912    -1.65   0.100    -46.32684
> 4.040777
>  _Idcode_11 |  -22.56438   9.922245    -2.27   0.023    -42.01162
> -3.117138
>  _Idcode_12 |  -12.55903   12.52187    -1.00   0.316    -37.10144
> 11.98338
>  _Idcode_13 |  -14.25003   9.233117    -1.54   0.123    -32.34661
> 3.846547
>  _Idcode_14 |  -26.27491   11.42556    -2.30   0.021    -48.66859
> -3.881229
>       _cons |   74.88709   53.28162     1.41   0.160    -29.54296
> 179.3171
> ------------------------------------------------------------------------
> ------
> Underidentification test (Kleibergen-Paap rk LM statistic):
> 1.825
>                                                   Chi-sq(1) P-val =
> 0.1767
> ------------------------------------------------------------------------
> ------
> Weak identification test (Cragg-Donald Wald F statistic):
> 2.977
>                         (Kleibergen-Paap rk Wald F statistic):
> 2.488
> Stock-Yogo weak ID test critical values: 10% maximal IV size
> 16.38
>                                         15% maximal IV size
> 8.96
>                                         20% maximal IV size
> 6.66
>                                         25% maximal IV size
> 5.53
> Source: Stock-Yogo (2005).  Reproduced by permission.
> NB: Critical values are for Cragg-Donald F statistic and i.i.d. errors.
> ------------------------------------------------------------------------
> ------
> Hansen J statistic (overidentification test of all instruments):
> 0.000
>                                                 (equation exactly
> identified)
> ------------------------------------------------------------------------
> ------
> Instrumented:         roaddum
> Included instruments: ims_hab_pop _Idcode_2 _Idcode_3 _Idcode_4
> _Idcode_5
>                      _Idcode_6 _Idcode_7 _Idcode_8 _Idcode_9 _Idcode_10
>                      _Idcode_11 _Idcode_12 _Idcode_13 _Idcode_14
> Excluded instruments: rank
> ------------------------------------------------------------------------
> ------
>
> Thanks,
> Shikha
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