请教一下我的系统面板两步回归，有两个解释变量TAANS TAACS和他们的交互项centerTAANCS， TAANS和TAACS目前我是作为外生变量处理，不知是否可以，还想问一下交互项是当内生变量还是外生变量呢？（两个都有试但是都不显著）
顺便请前辈们帮忙看一下我的命令是否有错误的地方？谢谢了
xtabond2 lnARRIVALD l.lnARRIVALD TAANS TAACS centerTAANCS lnGDP lnPOP  lnHIGHWAY year2003-year2012, gmm(lnARRIVALD,lag(2 3) collapse)  gmm(centerTAANCS, lag(2 3) collapse) iv(TAANS TAACS lnGDP lnPOP lnHIGHWAY year2003-year2012) twostep robust noconstant nodiffsargan
结果如下：

Dynamic panel-data estimation, two-step system GMM
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Group variable: city2                           Number of obs      =       410
Time variable : year                            Number of groups   =        41
Number of instruments = 21                      Obs per group: min =        10
Wald chi2(17) = 136803.24                                      avg =     10.00
Prob > chi2   =     0.000                                      max =        10
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             |              Corrected
  lnARRIVALD |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
  lnARRIVALD |
         L1. |   .4593768   .2738786     1.68   0.093    -.0774154    .9961689
             |
       TAANS |   .0927579   .0524608     1.77   0.077    -.0100635    .1955792
       TAACS |   .0396145   .0430599     0.92   0.358    -.0447813    .1240103
centerTAANCS |   -.006455   .0113592    -0.57   0.570    -.0287187    .0158087
       lnGDP |   .4113857   .1968005     2.09   0.037     .0256639    .7971076
       lnPOP |  -.3080697   .1893001    -1.63   0.104     -.679091    .0629517
   lnHIGHWAY |   .2926704   .1900867     1.54   0.124    -.0798927    .6652335
    year2003 |    2.95497   1.599717     1.85   0.065    -.1804175    6.090357
    year2004 |   3.129451    1.58641     1.97   0.049     .0201443    6.238757
    year2005 |   2.996303    1.58132     1.89   0.058    -.1030267    6.095632
    year2006 |   2.962353   1.530052     1.94   0.053    -.0364937    5.961199
    year2007 |   2.949607   1.526885     1.93   0.053    -.0430338    5.942247
    year2008 |   2.964401   1.547723     1.92   0.055    -.0690803    5.997882
    year2009 |   3.000552   1.574193     1.91   0.057    -.0848092    6.085913
    year2010 |   3.022044    1.57775     1.92   0.055    -.0702896    6.114378
    year2011 |   3.108088   1.604063     1.94   0.053    -.0358185    6.251994
    year2012 |   3.165432   1.650897     1.92   0.055    -.0702657     6.40113
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Instruments for first differences equation
  Standard
    D.(TAANS TAACS lnGDP lnPOP lnHIGHWAY year2003 year2004 year2005 year2006
    year2007 year2008 year2009 year2010 year2011 year2012)
  GMM-type (missing=0, separate instruments for each period unless collapsed)
    L(2/3).centerTAANCS collapsed
    L(2/3).lnARRIVALD collapsed
Instruments for levels equation
  Standard
    TAANS TAACS lnGDP lnPOP lnHIGHWAY year2003 year2004 year2005 year2006
    year2007 year2008 year2009 year2010 year2011 year2012
  GMM-type (missing=0, separate instruments for each period unless collapsed)
    DL.centerTAANCS collapsed
    DL.lnARRIVALD collapsed
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Arellano-Bond test for AR(1) in first differences: z =  -2.24  Pr > z =  0.025
Arellano-Bond test for AR(2) in first differences: z =   1.28  Pr > z =  0.201
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Sargan test of overid. restrictions: chi2(4)    =  25.24  Prob > chi2 =  0.000
  (Not robust, but not weakened by many instruments.)
Hansen test of overid. restrictions: chi2(4)    =   8.95  Prob > chi2 =  0.062
  (Robust, but weakened by many instruments.)