先贴一下回归结果：Dynamic panel-data estimation, one-step system GMM
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Group variable: n                               Number of obs      =      3004
Time variable : nbsj                            Number of groups   =       751
Number of instruments = 23                      Obs per group: min =         4
F(8, 750)     =      7.71                                      avg =      4.00
Prob > F      =     0.000                                      max =         4
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             |               Robust
       rd_in |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
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       rd_in |
         L1. |   .5265644   .1316515     4.00   0.000     .2681151    .7850137
             |
          ex |   -.271882   .0836754    -3.25   0.001    -.4361478   -.1076162
         gov |   .1294468   .0500411     2.59   0.010     .0312095    .2276841
       size2 |  -.0093836   .0028761    -3.26   0.001    -.0150297   -.0037374
         age |  -.0007376   .0004956    -1.49   0.137    -.0017106    .0002353
          da |   .0019441   .0004658     4.17   0.000     .0010298    .0028585
        lrl3 |   -.064853   .0238749    -2.72   0.007    -.1117225   -.0179834
          jg |   .0098909   .0069517     1.42   0.155    -.0037563    .0235381
       _cons |   .2892573   .0773888     3.74   0.000     .1373329    .4411816
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Instruments for first differences equation
  Standard
    D.(gov size2 age da lrl3 jg)
  GMM-type (missing=0, separate instruments for each period unless collapsed)
    L(1/4).L.ex
    L.rd_in
Instruments for levels equation
  Standard
    gov size2 age da lrl3 jg
    _cons
  GMM-type (missing=0, separate instruments for each period unless collapsed)
    D.L.ex
    D.rd_in
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Arellano-Bond test for AR(1) in first differences: z =  -2.74  Pr > z =  0.006
Arellano-Bond test for AR(2) in first differences: z =   0.21  Pr > z =  0.833
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Sargan test of overid. restrictions: chi2(14)   =1770.47  Prob > chi2 =  0.000
  (Not robust, but not weakened by many instruments.)
Hansen test of overid. restrictions: chi2(14)   =  59.95  Prob > chi2 =  0.000
  (Robust, but weakened by many instruments.)

Difference-in-Hansen tests of exogeneity of instrument subsets:
  GMM instruments for levels
    Hansen test excluding group:     chi2(7)    =  12.98  Prob > chi2 =  0.072
    Difference (null H = exogenous): chi2(7)    =  46.97  Prob > chi2 =  0.000
  gmm(rd_in, lag(1 1))
    Hansen test excluding group:     chi2(7)    =   5.93  Prob > chi2 =  0.548
    Difference (null H = exogenous): chi2(7)    =  54.02  Prob > chi2 =  0.000
  gmm(L.ex, lag(1 .))
    Hansen test excluding group:     chi2(5)    =  52.72  Prob > chi2 =  0.000
    Difference (null H = exogenous): chi2(9)    =   7.23  Prob > chi2 =  0.613
  iv(gov size2 age da lrl3 jg)
    Hansen test excluding group:     chi2(8)    =  17.96  Prob > chi2 =  0.022
    Difference (null H = exogenous): chi2(6)    =  41.99  Prob > chi2 =  0.000


有这么几个问题：
第一、sargan检验通不过，是因为工具变量太多了吗？
第二、xtabond2命令中，gmm()和iv()这两类工具变量是按照什么原则区分的？
第三、Difference-in-Hansen tests的结果应该怎么分析呢？
第四、是否使用twostep？应该根据什么判断？

刚刚接触系统GMM，问题比较多=。=提前谢过各位！