我输入的命令是：xtabond2 lnpri lnl1 bubble lnincome lnland newinterest lnCPI, gmm(lnl1 bubble) iv(lnincome lnland newinterest lnCPI) robust twostep small
问题一：我的解释变量bubble中有(Yt-1—Yt-2)/Yt-2的滞后一期和滞后两期Y同时出现的情况，那么照上述命令做GMM估计有问题吗？
问题二：lnincome lnland newinterest三个解释变量在5%显著性水平上都不显著，有没有什么较好的改进方法？（income指可支配收入 land土地价格 interest名义利率）
问题三：怎么看Sargan test，Hansen test和Arellano-Bond test的结果？
刚开始接触计量和stata，希望大家不吝赐教！！！

Dynamic panel-data estimation, two-step system GMM
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Group variable: id                              Number of obs      =       210
Time variable : year                            Number of groups   =        30
Number of instruments = 66                      Obs per group: min =         7
F(6, 29)      =   1066.25                                      avg =      7.00
Prob > F      =     0.000                                      max =         7
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             |              Corrected
       lnpri |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
        lnl1 |   .9932269   .0665125    14.93   0.000     .8571937     1.12926
      bubble |  -.1613536    .062633    -2.58   0.015    -.2894525   -.0332547
    lnincome |   .0107365   .0974249     0.11   0.913    -.1885198    .2099929
      lnland |   .0119812   .0172494     0.69   0.493    -.0232979    .0472602
newinterest |   1.628302   .8220475     1.98   0.057    -.0529745    3.309578
       lnCPI |  -.8616169   .2387933    -3.61   0.001    -1.350004   -.3732298
       _cons |   3.898482   1.049357     3.72   0.001     1.752307    6.044658
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Instruments for first differences equation
  Standard
    D.(lnincome lnland newinterest lnCPI)
  GMM-type (missing=0, separate instruments for each period unless collapsed)
    L(1/8).(lnl1 bubble)
Instruments for levels equation
  Standard
    lnincome lnland newinterest lnCPI
    _cons
  GMM-type (missing=0, separate instruments for each period unless collapsed)
    D.(lnl1 bubble)
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Arellano-Bond test for AR(1) in first differences: z =  -2.94  Pr > z =  0.003
Arellano-Bond test for AR(2) in first differences: z =   1.56  Pr > z =  0.119
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Sargan test of overid. restrictions: chi2(59)   =  95.63  Prob > chi2 =  0.002
  (Not robust, but not weakened by many instruments.)
Hansen test of overid. restrictions: chi2(59)   =  26.52  Prob > chi2 =  1.000
  (Robust, but weakened by many instruments.)

Difference-in-Hansen tests of exogeneity of instrument subsets:
  GMM instruments for levels
    Hansen test excluding group:     chi2(46)   =  24.02  Prob > chi2 =  0.997
    Difference (null H = exogenous): chi2(13)   =   2.50  Prob > chi2 =  0.999
  iv(lnincome lnland newinterest lnCPI)
    Hansen test excluding group:     chi2(55)   =  26.99  Prob > chi2 =  0.999
    Difference (null H = exogenous): chi2(4)    =  -0.47  Prob > chi2 =  1.000