xtabond2 tech l.tech zf yf qy gdp, gmm(zf yf qy gdp,lag(1 2)) iv(zf yf qy gdp)
>  twostep
Favoring space over speed. To switch, type or click on mata: mata set matafavor
> speed, perm.
Warning: Number of instruments may be large relative to number of observations.
Warning: Two-step estimated covariance matrix of moments is singular.
  Using a generalized inverse to calculate optimal weighting matrix for two-step
>  estimation.
  Difference-in-Sargan/Hansen statistics may be negative.

Dynamic panel-data estimation, two-step system GMM
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Group variable: dq                              Number of obs      =        55
Time variable : year                            Number of groups   =        11
Number of instruments = 55                      Obs per group: min =         5
Wald chi2(5)  =      7.78                                      avg =      5.00
Prob > chi2   =     0.169                                      max =         5
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        tech |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
        tech |
         L1. |  -.4689717   .2106668    -2.23   0.026    -.8818711   -.0560723
             |
          zf |   .0913973   .0565394     1.62   0.096    -.0194179    .2022126
          yf |   .2227561   .1200492     1.86   0.064     -.012536    .4580483
          qy |   .2206338   .1711537     1.29   0.197    -.1148213    .5560889
         gdp |  -.8720008   .5268164    -1.66   0.098    -1.904542    .1605403
       _cons |   6.081684   3.519503     1.73   0.084    -.8164142    12.97978
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Warning: Uncorrected two-step standard errors are unreliable.

Instruments for first differences equation
  Standard
    D.(zf yf qy gdp)
  GMM-type (missing=0, separate instruments for each period unless collapsed)
    L(1/2).(zf yf qy gdp)
Instruments for levels equation
  Standard
    zf yf qy gdp
    _cons
  GMM-type (missing=0, separate instruments for each period unless collapsed)
    D.(zf yf qy gdp)
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Arellano-Bond test for AR(1) in first differences: z =   0.33  Pr > z =  0.739
Arellano-Bond test for AR(2) in first differences: z =   0.41  Pr > z =  0.679
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Sargan test of overid. restrictions: chi2(49)   =  72.97  Prob > chi2 =  0.015
  (Not robust, but not weakened by many instruments.)
Hansen test of overid. restrictions: chi2(49)   =   3.47  Prob > chi2 =  1.000
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