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st: F-stat of excl inst too big in robsut estimation with dummydependent variable (xtivreg2)


From   mparrat@bu.edu
To   statalist@hsphsun2.harvard.edu
Subject   st: F-stat of excl inst too big in robsut estimation with dummydependent variable (xtivreg2)
Date   Tue, 01 Apr 2008 01:44:03 -0400

Hi,

I need some help understanding why the F-statistic of the excluded instruments is so big for a subset of my sample.

I am estimating an IV fe model using xtivreg: y x1 x2 t ct (x1=z), fe robust fwl(t ct) i(nidhogar) t(year). My full sample is composed by 1334 households and 9 years (10886 obs. in total). The dependent variable is a dummy taking the value of 1 if the household owns a business and zero otherwise. The endogenous variable is an income variable truncated at zero for all households but taking positive values for some households in all or some years. In addition I am controlling for time-varying household characteristics, time dummies and community-time dummies (there are 8 communities- too few to cluster on). I use the fwl option to partial out the time and community-time dummies. The instrument is a variable similar to the endogenous variable, that is, an income type variable but exogenous. I am confident about my instrument doing a good job in correcting for endogeneity. Also, in previous versions I had included additional instruments and tested for overidentification, endogeneity and redundancy and this one performed quite well.

Looking at the results of the full sample, I believe the estimation is just fine. My concern is that when I partition the sample in two (household heads younger and older than 39 years old) the F-statistic of excluded instruments becomes very large for the sample of younger heads (5712.73). However, standard errors in both first stage and main regression and other statistics seem fine. What could be happening? What am I missing? Are my results still valid? How can I fix this, if there is something to fix?

I read an earlier post (by Vince Wiggins answering to Mark Schaffer) in which the problem was the F-stat not being reported in a heteroskedastic-robust estimation with dummy variables. My intuition is that in my case the opposite might be happening, but not sure? I checked and my dependent variable takes the value of 1 for 23% of the observations in the sample of young heads.

The sample size for the two age groups is: 3477 observations (591 hhs) and 7727 (1031 hhs).

Below I?m copying the results of the estimations using the full sample and the restricted samples. Please let me know if further information is needed.

I very much appreciate your comments and suggestions.

Regards,

Monica



FULL SAMPLE:

. xtivreg2 dabus_sum

dagro lnpland_manz `agesq' `hhcharact' `ctfe' `te' ( potincm_fo = `inst80s_fo' )
, fe i(nidhogar) r first ffirst rf fwl(`ctfe' `te') ;
Warning - singleton groups detected.  38 observation(s) not used.
Warning - collinearities detected
Vars dropped:  dcomm1_99 dcomm2_98 dcomm3_99 dcomm4_96 dcomm5_94 dcomm7_98
               dcomm8_99 dyr9

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =      1334                    Obs per group: min =         2
                                                               avg =       8.2
                                                               max =         9

Reduced-form regression: dabus_sum
----------------------------------

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =      1334                    Obs per group: min =         2
                                                               avg =       8.2
                                                               max =         9

OLS estimation
--------------

Statistics robust to heteroskedasticity

                                                      Number of obs =    10886
                                                      F(  6,  9546) =    11.03
                                                      Prob > F      =   0.0000
Total (centered) SS     =  280.0052317                Centered R2   =   0.0155
Total (uncentered) SS   =  280.0052317                Uncentered R2 =   0.0155
Residual SS             =  275.6691323                Root MSE      =    .1699

------------------------------------------------------------------------------
             |               Robust
   dabus_sum |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
       dagro |  -.1568444   .0304295    -5.15   0.000    -.2164927   -.0971961
lnpland_manz |  -.0091142   .0113462    -0.80   0.422    -.0313552    .0131269
     agehead |    .024494   .0045082     5.43   0.000     .0156569    .0333311
  ageheadsqr |  -.0001439   .0000387    -3.72   0.000    -.0002198   -.0000681
  kids14less |    .003819    .003182     1.20   0.230    -.0024184    .0100565
potincm80s~o |  -.0143489   .0072045    -1.99   0.046    -.0284713   -.0002265
------------------------------------------------------------------------------
Included instruments: dagro lnpland_manz agehead ageheadsqr kids14less
                      potincm80s_fo
------------------------------------------------------------------------------


First-stage regressions
-----------------------

First-stage regression of potincm_fo:

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =      1334                    Obs per group: min =         2
                                                               avg =       8.2
                                                               max =         9

OLS estimation
--------------

Statistics robust to heteroskedasticity

                                                      Number of obs =    10886
                                                      F(  6,  9546) =    16.47
                                                      Prob > F      =   0.0000
Total (centered) SS     =  10371.54444                Centered R2   =   0.0902
Total (uncentered) SS   =  10371.54444                Uncentered R2 =   0.0902
Residual SS             =  9436.014852                Root MSE      =    .9942

------------------------------------------------------------------------------
             |               Robust
  potincm_fo |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
       dagro |    .031293   .0752245     0.42   0.677    -.1161631    .1787491
lnpland_manz |  -.0771407   .0256491    -3.01   0.003    -.1274185    -.026863
     agehead |  -.0498908   .0247468    -2.02   0.044    -.0983999   -.0013818
  ageheadsqr |   .0011953    .000256     4.67   0.000     .0006935    .0016972
  kids14less |  -.0290633   .0182759    -1.59   0.112    -.0648879    .0067613
potincm80s~o |   .6552578   .1159389     5.65   0.000     .4279929    .8825227
------------------------------------------------------------------------------
Included instruments: dagro lnpland_manz agehead ageheadsqr kids14less
                      potincm80s_fo
------------------------------------------------------------------------------
Partial R-squared of excluded instruments:   0.0839
Test of excluded instruments:
  F(  1,  9546) =    31.94
  Prob > F      =   0.0000



Summary results for first-stage regressions
-------------------------------------------

Variable    | Shea Partial R2 |   Partial R2    |  F(  1,  9546)    P-value
potincm_fo  |     0.0839      |     0.0839      |      31.94        0.0000

NB: first-stage F-stat heteroskedasticity-robust

Underidentification tests
Ho: matrix of reduced form coefficients has rank=K-1 (underidentified)
Ha: matrix has rank=K (identified)
                                                 Chi-sq(1)      P-value
Anderson canon. corr. -N*ln(1-CCEV) LR stat.      837.05         0.0000
Cragg-Donald N*CDEV statistic                     874.82         0.0000
Robust chi-square statistic                        31.96         0.0000

Weak identification tests
Ho: equation is weakly identified
Cragg-Donald (N-L)*CDEV/L1 F-statistic            874.27
Robust F-statistic                                 31.94
See main output for Cragg-Donald weak id test critical values

NB: Anderson and Cragg-Donald under- and weak identification stats not robust
    Robust identification stats heteroskedasticity-robust

Weak-instrument-robust inference
Tests of joint significance of endogenous regressors B1 in main equation
Ho: B1=0 and overidentifying restrictions are valid
Anderson-Rubin test          F(1,9546)=3.97      P-val=0.0464
Anderson-Rubin test          Chi-sq(1)=3.97      P-val=0.0463
Stock-Wright S statistic     Chi-sq(1)=2.98      P-val=0.0845
NB: Test statistics heteroskedasticity-robust

Number of observations N           =      10886
Number of regressors   K           =          6
Number of instruments  L           =          6
Number of excluded instruments L1  =          1


IV (2SLS) estimation
--------------------

Statistics robust to heteroskedasticity

                                                      Number of obs =    10886
                                                      F(  6,  9546) =    11.10
                                                      Prob > F      =   0.0000
Total (centered) SS     =  280.0052317                Centered R2   =   0.0008
Total (uncentered) SS   =  280.0052317                Uncentered R2 =   0.0008
Residual SS             =  279.7700696                Root MSE      =    .1711

------------------------------------------------------------------------------
             |               Robust
   dabus_sum |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
  potincm_fo |  -.0218981   .0100669    -2.18   0.030    -.0416289   -.0021673
       dagro |  -.1561592   .0302779    -5.16   0.000    -.2155027   -.0968156
lnpland_manz |  -.0108034    .010944    -0.99   0.324    -.0322532    .0106464
     agehead |   .0234015   .0046015     5.09   0.000     .0143827    .0324203
  ageheadsqr |  -.0001177   .0000403    -2.92   0.003    -.0001967   -.0000388
  kids14less |   .0031826   .0032288     0.99   0.324    -.0031456    .0095109
------------------------------------------------------------------------------
Anderson canon. corr. LR statistic (underidentification test):         837.049
                                                   Chi-sq(1) P-val =    0.0000
Test statistic(s) not robust
------------------------------------------------------------------------------
Cragg-Donald F statistic (weak identification test):                   874.270
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
Test statistic(s) not robust
Source: Stock-Yogo (2005).  Reproduced by permission.
------------------------------------------------------------------------------
Hansen J statistic (overidentification test of all instruments):         0.000
                                                 (equation exactly identified)
------------------------------------------------------------------------------
Instrumented:         potincm_fo
Included instruments: dagro lnpland_manz agehead ageheadsqr kids14less
Excluded instruments: potincm80s_fo
Partialled-out (FWL): dcomm1_91 dcomm1_92 dcomm1_93 dcomm1_94 dcomm1_95
                      dcomm1_96 dcomm1_97 dcomm1_98 dcomm2_91 dcomm2_92
                      dcomm2_93 dcomm2_94 dcomm2_95 dcomm2_96 dcomm2_97
                      dcomm2_99 dcomm3_91 dcomm3_92 dcomm3_93 dcomm3_94
                      dcomm3_95 dcomm3_96 dcomm3_97 dcomm3_98 dcomm4_91
                      dcomm4_92 dcomm4_93 dcomm4_94 dcomm4_95 dcomm4_97
                      dcomm4_98 dcomm4_99 dcomm5_91 dcomm5_92 dcomm5_93
                      dcomm5_95 dcomm5_96 dcomm5_97 dcomm5_98 dcomm5_99
                      dcomm7_91 dcomm7_92 dcomm7_93 dcomm7_94 dcomm7_95
                      dcomm7_96 dcomm7_97 dcomm7_99 dcomm8_91 dcomm8_92
                      dcomm8_93 dcomm8_94 dcomm8_95 dcomm8_96 dcomm8_97
                      dcomm8_98 dyr2 dyr3 dyr4 dyr5 dyr6 dyr7 dyr8
                      nb: variable counts and small-sample adjustments
                      do not include partialled-out variables.
Dropped collinear:    dcomm1_99 dcomm2_98 dcomm3_99 dcomm4_96 dcomm5_94
                      dcomm7_98 dcomm8_99 dyr9
------------------------------------------------------------------------------






YOUNGER HEADS SAMPLE:



. xtivreg2 dabus_sum
dagro lnpland_manz `agesq' `hhcharact' `ctfe' `te' ( potincm_fo = `inst80s_fo' )
if dagehead2539 ==1 , fe i(nidhogar) r first ffirst rf fwl(`ctfe' `te') ;
Warning - singleton groups detected.  69 observation(s) not used.
Warning - collinearities detected
Vars dropped:  dcomm1_99 dcomm2_99 dcomm3_99 dcomm4_99 dcomm5_97 dcomm7_98
               dcomm8_99 dyr9

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =       749                    Obs per group: min =         2
                                                               avg =       6.6
                                                               max =         9

Reduced-form regression: dabus_sum
----------------------------------

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =       749                    Obs per group: min =         2
                                                               avg =       6.6
                                                               max =         9

OLS estimation
--------------

Statistics robust to heteroskedasticity

                                                      Number of obs =     4959
                                                      F(  6,  4204) =     7.23
                                                      Prob > F      =   0.0000
Total (centered) SS     =  119.6871152                Centered R2   =   0.0322
Total (uncentered) SS   =  119.6871152                Uncentered R2 =   0.0322
Residual SS             =  115.8295136                Root MSE      =     .166

------------------------------------------------------------------------------
             |               Robust
   dabus_sum |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
       dagro |  -.1429524   .0403911    -3.54   0.000    -.2221404   -.0637645
lnpland_manz |  -.0025269   .0153996    -0.16   0.870    -.0327183    .0276645
     agehead |   .0358974   .0136655     2.63   0.009     .0091058    .0626891
  ageheadsqr |  -.0003511   .0002082    -1.69   0.092    -.0007592     .000057
  kids14less |   .0056506   .0051194     1.10   0.270    -.0043861    .0156873
potincm80s~o |  -.0641757   .0172277    -3.73   0.000    -.0979511   -.0304002
------------------------------------------------------------------------------
Included instruments: dagro lnpland_manz agehead ageheadsqr kids14less
                      potincm80s_fo
------------------------------------------------------------------------------


First-stage regressions
-----------------------

First-stage regression of potincm_fo:

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =       749                    Obs per group: min =         2
                                                               avg =       6.6
                                                               max =         9

OLS estimation
--------------

Statistics robust to heteroskedasticity

                                                      Number of obs =     4959
                                                      F(  6,  4204) =  1000.42
                                                      Prob > F      =   0.0000
Total (centered) SS     =  1882.108159                Centered R2   =   0.3080
Total (uncentered) SS   =  1882.108159                Uncentered R2 =   0.3080
Residual SS             =   1302.35443                Root MSE      =    .5566

------------------------------------------------------------------------------
             |               Robust
  potincm_fo |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
       dagro |   .0813442    .070368     1.16   0.248    -.0566143    .2193027
lnpland_manz |  -.0382054   .0165208    -2.31   0.021     -.070595   -.0058159
     agehead |   .1004971    .040134     2.50   0.012     .0218132    .1791809
  ageheadsqr |  -.0009395   .0006719    -1.40   0.162    -.0022568    .0003778
  kids14less |  -.0212255   .0166119    -1.28   0.201    -.0537936    .0113425
potincm80s~o |   .9976588   .0131996    75.58   0.000     .9717807    1.023537
------------------------------------------------------------------------------
Included instruments: dagro lnpland_manz agehead ageheadsqr kids14less
                      potincm80s_fo
------------------------------------------------------------------------------
Partial R-squared of excluded instruments:   0.3043
Test of excluded instruments:
  F(  1,  4204) =  5712.73
  Prob > F      =   0.0000



Summary results for first-stage regressions
-------------------------------------------

Variable    | Shea Partial R2 |   Partial R2    |  F(  1,  4204)    P-value
potincm_fo  |     0.3043      |     0.3043      |    5712.73        0.0000

NB: first-stage F-stat heteroskedasticity-robust

Underidentification tests
Ho: matrix of reduced form coefficients has rank=K-1 (underidentified)
Ha: matrix has rank=K (identified)
                                                 Chi-sq(1)      P-value
Anderson canon. corr. -N*ln(1-CCEV) LR stat.     1527.72         0.0000
Cragg-Donald N*CDEV statistic                    1841.71         0.0000
Robust chi-square statistic                      5720.89         0.0000

Weak identification tests
Ho: equation is weakly identified
Cragg-Donald (N-L)*CDEV/L1 F-statistic           1839.09
Robust F-statistic                               5712.73
See main output for Cragg-Donald weak id test critical values

NB: Anderson and Cragg-Donald under- and weak identification stats not robust
    Robust identification stats heteroskedasticity-robust

Weak-instrument-robust inference
Tests of joint significance of endogenous regressors B1 in main equation
Ho: B1=0 and overidentifying restrictions are valid
Anderson-Rubin test          F(1,4204)=13.88     P-val=0.0002
Anderson-Rubin test          Chi-sq(1)=13.90     P-val=0.0002
Stock-Wright S statistic     Chi-sq(1)=5.61      P-val=0.0178
NB: Test statistics heteroskedasticity-robust

Number of observations N           =       4959
Number of regressors   K           =          6
Number of instruments  L           =          6
Number of excluded instruments L1  =          1


IV (2SLS) estimation
--------------------

Statistics robust to heteroskedasticity

                                                      Number of obs =     4959
                                                      F(  6,  4204) =     6.80
                                                      Prob > F      =   0.0000
Total (centered) SS     =  119.6871152                Centered R2   =   0.0067
Total (uncentered) SS   =  119.6871152                Uncentered R2 =   0.0067
Residual SS             =  118.8838886                Root MSE      =     .168

------------------------------------------------------------------------------
             |               Robust
   dabus_sum |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
  potincm_fo |  -.0643263   .0173661    -3.70   0.000    -.0983631   -.0302894
       dagro |  -.1377199   .0407396    -3.38   0.001     -.217568   -.0578717
lnpland_manz |  -.0049845   .0154831    -0.32   0.748    -.0353308    .0253617
     agehead |   .0423621   .0139741     3.03   0.002     .0149734    .0697507
  ageheadsqr |  -.0004116   .0002127    -1.94   0.053    -.0008284    5.30e-06
  kids14less |   .0042853   .0051692     0.83   0.407    -.0058461    .0144167
------------------------------------------------------------------------------
Anderson canon. corr. LR statistic (underidentification test):        1527.719
                                                   Chi-sq(1) P-val =    0.0000
Test statistic(s) not robust
------------------------------------------------------------------------------
Cragg-Donald F statistic (weak identification test):                  1839.088
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
Test statistic(s) not robust
Source: Stock-Yogo (2005).  Reproduced by permission.
------------------------------------------------------------------------------
Hansen J statistic (overidentification test of all instruments):         0.000
                                                 (equation exactly identified)
------------------------------------------------------------------------------
Instrumented:         potincm_fo
Included instruments: dagro lnpland_manz agehead ageheadsqr kids14less
Excluded instruments: potincm80s_fo
Partialled-out (FWL): dcomm1_91 dcomm1_92 dcomm1_93 dcomm1_94 dcomm1_95
                      dcomm1_96 dcomm1_97 dcomm1_98 dcomm2_91 dcomm2_92
                      dcomm2_93 dcomm2_94 dcomm2_95 dcomm2_96 dcomm2_97
                      dcomm2_98 dcomm3_91 dcomm3_92 dcomm3_93 dcomm3_94
                      dcomm3_95 dcomm3_96 dcomm3_97 dcomm3_98 dcomm4_91
                      dcomm4_92 dcomm4_93 dcomm4_94 dcomm4_95 dcomm4_96
                      dcomm4_97 dcomm4_98 dcomm5_91 dcomm5_92 dcomm5_93
                      dcomm5_94 dcomm5_95 dcomm5_96 dcomm5_98 dcomm5_99
                      dcomm7_91 dcomm7_92 dcomm7_93 dcomm7_94 dcomm7_95
                      dcomm7_96 dcomm7_97 dcomm7_99 dcomm8_91 dcomm8_92
                      dcomm8_93 dcomm8_94 dcomm8_95 dcomm8_96 dcomm8_97
                      dcomm8_98 dyr2 dyr3 dyr4 dyr5 dyr6 dyr7 dyr8
                      nb: variable counts and small-sample adjustments
                      do not include partialled-out variables.
Dropped collinear:    dcomm1_99 dcomm2_99 dcomm3_99 dcomm4_99 dcomm5_97
                      dcomm7_98 dcomm8_99 dyr9
------------------------------------------------------------------------------







OLDER HEADS SAMPLE :


. xtivreg2 dabus_sum
dagro lnpland_manz `agesq' `hhcharact' `ctfe' `te' ( potincm_fo = `inst80s_fo' )
if dagehead4065 ==1 , fe i(nidhogar) r first ffirst rf fwl(`ctfe' `te') ;
Warning - singleton groups detected.  56 observation(s) not used.
Warning - collinearities detected
Vars dropped:  dcomm1_99 dcomm2_95 dcomm3_96 dcomm4_97 dcomm5_98 dcomm7_99
               dcomm8_99 dyr9

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =       830                    Obs per group: min =         2
                                                               avg =       7.0
                                                               max =         9

Reduced-form regression: dabus_sum
----------------------------------

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =       830                    Obs per group: min =         2
                                                               avg =       7.0
                                                               max =         9

OLS estimation
--------------

Statistics robust to heteroskedasticity

                                                      Number of obs =     5840
                                                      F(  6,  5004) =     9.95
                                                      Prob > F      =   0.0000
Total (centered) SS     =  130.6892353                Centered R2   =   0.0200
Total (uncentered) SS   =  130.6892353                Uncentered R2 =   0.0200
Residual SS             =  128.0706507                Root MSE      =      .16

------------------------------------------------------------------------------
             |               Robust
   dabus_sum |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
       dagro |  -.1704666   .0498948    -3.42   0.001    -.2682824   -.0726509
lnpland_manz |  -.0023589   .0059152    -0.40   0.690    -.0139553    .0092375
     agehead |   .0638039   .0102197     6.24   0.000     .0437689    .0838389
  ageheadsqr |  -.0005301   .0000889    -5.97   0.000    -.0007043   -.0003559
  kids14less |   .0099594   .0049148     2.03   0.043     .0003242    .0195947
potincm80s~o |   .0093771   .0038328     2.45   0.014     .0018631    .0168911
------------------------------------------------------------------------------
Included instruments: dagro lnpland_manz agehead ageheadsqr kids14less
                      potincm80s_fo
------------------------------------------------------------------------------


First-stage regressions
-----------------------

First-stage regression of potincm_fo:

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =       830                    Obs per group: min =         2
                                                               avg =       7.0
                                                               max =         9

OLS estimation
--------------

Statistics robust to heteroskedasticity

                                                      Number of obs =     5840
                                                      F(  6,  5004) =     7.64
                                                      Prob > F      =   0.0000
Total (centered) SS     =  8142.451572                Centered R2   =   0.0463
Total (uncentered) SS   =  8142.451572                Uncentered R2 =   0.0463
Residual SS             =  7765.373403                Root MSE      =    1.246

------------------------------------------------------------------------------
             |               Robust
  potincm_fo |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
       dagro |   .0590806   .1640592     0.36   0.719    -.2625472    .3807085
lnpland_manz |  -.1176091   .0390987    -3.01   0.003    -.1942598   -.0409584
     agehead |   .0332028   .0682443     0.49   0.627    -.1005861    .1669916
  ageheadsqr |   .0003195   .0006371     0.50   0.616    -.0009294    .0015684
  kids14less |  -.0352024    .031388    -1.12   0.262    -.0967366    .0263317
potincm80s~o |   .5135334   .1413639     3.63   0.000     .2363982    .7906685
------------------------------------------------------------------------------
Included instruments: dagro lnpland_manz agehead ageheadsqr kids14less
                      potincm80s_fo
------------------------------------------------------------------------------
Partial R-squared of excluded instruments:   0.0446
Test of excluded instruments:
  F(  1,  5004) =    13.20
  Prob > F      =   0.0003



Summary results for first-stage regressions
-------------------------------------------

Variable    | Shea Partial R2 |   Partial R2    |  F(  1,  5004)    P-value
potincm_fo  |     0.0446      |     0.0446      |      13.20        0.0003

NB: first-stage F-stat heteroskedasticity-robust

Underidentification tests
Ho: matrix of reduced form coefficients has rank=K-1 (underidentified)
Ha: matrix has rank=K (identified)
                                                 Chi-sq(1)      P-value
Anderson canon. corr. -N*ln(1-CCEV) LR stat.      228.47         0.0000
Cragg-Donald N*CDEV statistic                     233.76         0.0000
Robust chi-square statistic                        13.21         0.0003

Weak identification tests
Ho: equation is weakly identified
Cragg-Donald (N-L)*CDEV/L1 F-statistic            233.48
Robust F-statistic                                 13.20
See main output for Cragg-Donald weak id test critical values

NB: Anderson and Cragg-Donald under- and weak identification stats not robust
    Robust identification stats heteroskedasticity-robust

Weak-instrument-robust inference
Tests of joint significance of endogenous regressors B1 in main equation
Ho: B1=0 and overidentifying restrictions are valid
Anderson-Rubin test          F(1,5004)=5.99      P-val=0.0145
Anderson-Rubin test          Chi-sq(1)=5.99      P-val=0.0144
Stock-Wright S statistic     Chi-sq(1)=6.48      P-val=0.0109
NB: Test statistics heteroskedasticity-robust

Number of observations N           =       5840
Number of regressors   K           =          6
Number of instruments  L           =          6
Number of excluded instruments L1  =          1


IV (2SLS) estimation
--------------------

Statistics robust to heteroskedasticity

                                                      Number of obs =     5840
                                                      F(  6,  5004) =     9.91
                                                      Prob > F      =   0.0000
Total (centered) SS     =  130.6892353                Centered R2   =   0.0038
Total (uncentered) SS   =  130.6892353                Uncentered R2 =   0.0038
Residual SS             =  130.1981904                Root MSE      =    .1612

------------------------------------------------------------------------------
             |               Robust
   dabus_sum |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
  potincm_fo |   .0182599   .0079422     2.30   0.021     .0026936    .0338263
       dagro |  -.1715454   .0502306    -3.42   0.001    -.2699957   -.0730952
lnpland_manz |  -.0002114   .0060272    -0.04   0.972    -.0120245    .0116017
     agehead |   .0631976   .0102148     6.19   0.000      .043177    .0832183
  ageheadsqr |   -.000536   .0000894    -6.00   0.000    -.0007111   -.0003608
  kids14less |   .0106022   .0049372     2.15   0.032     .0009254     .020279
------------------------------------------------------------------------------
Anderson canon. corr. LR statistic (underidentification test):         228.468
                                                   Chi-sq(1) P-val =    0.0000
Test statistic(s) not robust
------------------------------------------------------------------------------
Cragg-Donald F statistic (weak identification test):                   233.478
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
Test statistic(s) not robust
Source: Stock-Yogo (2005).  Reproduced by permission.
------------------------------------------------------------------------------
Hansen J statistic (overidentification test of all instruments):         0.000
                                                 (equation exactly identified)
------------------------------------------------------------------------------
Instrumented:         potincm_fo
Included instruments: dagro lnpland_manz agehead ageheadsqr kids14less
Excluded instruments: potincm80s_fo
Partialled-out (FWL): dcomm1_91 dcomm1_92 dcomm1_93 dcomm1_94 dcomm1_95
                      dcomm1_96 dcomm1_97 dcomm1_98 dcomm2_91 dcomm2_92
                      dcomm2_93 dcomm2_94 dcomm2_96 dcomm2_97 dcomm2_98
                      dcomm2_99 dcomm3_91 dcomm3_92 dcomm3_93 dcomm3_94
                      dcomm3_95 dcomm3_97 dcomm3_98 dcomm3_99 dcomm4_91
                      dcomm4_92 dcomm4_93 dcomm4_94 dcomm4_95 dcomm4_96
                      dcomm4_98 dcomm4_99 dcomm5_91 dcomm5_92 dcomm5_93
                      dcomm5_94 dcomm5_95 dcomm5_96 dcomm5_97 dcomm5_99
                      dcomm7_91 dcomm7_92 dcomm7_93 dcomm7_94 dcomm7_95
                      dcomm7_96 dcomm7_97 dcomm7_98 dcomm8_91 dcomm8_92
                      dcomm8_93 dcomm8_94 dcomm8_95 dcomm8_96 dcomm8_97
                      dcomm8_98 dyr2 dyr3 dyr4 dyr5 dyr6 dyr7 dyr8
                      nb: variable counts and small-sample adjustments
                      do not include partialled-out variables.
Dropped collinear:    dcomm1_99 dcomm2_95 dcomm3_96 dcomm4_97 dcomm5_98
                      dcomm7_99 dcomm8_99 dyr9
------------------------------------------------------------------------------






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