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出现下面的情况，是为什么呀？
xtabond2 lnco22 l.lnco22 lnopen lnei lngdp lnes lntechnology i.year, gmm(lnco22,lag(1 2) collapse) gmm(lnopen lnei,lag(1 2) c> ollapse) iv ( lngdp lnes lntechnology i.year) twostep robust 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------------------------------------------------------------------------------Group variable: name Number of obs = 523Time variable : year Number of groups = 28Number of instruments = 32 Obs per group: min = 13Wald chi2(31) = 92843.03 avg = 18.68Prob > chi2 = 0.000 max = 20------------------------------------------------------------------------------ | Corrected lnco22 | Coef. Std. Err. z P>|z| [95% Conf. Interval]-------------+---------------------------------------------------------------- lnco22 | L1. | .0909529 .1308283 0.70 0.487 -.1654658 .3473717 | lnopen | .1115317 .1027489 1.09 0.278 -.0898524 .3129158 lnei | .589919 .2912548 2.03 0.043 .0190701 1.160768 lngdp | -.4310216 .1502749 -2.87 0.004 -.725555 -.1364883 lnes | .454572 .5000842 0.91 0.363 -.525575 1.434719lntechnology | -.0418072 .0800447 -0.52 0.601 -.1986919 .1150775 | year | 1992 | 0 (empty) 1993 | 0 (omitted) 1994 | 0 (omitted) 1995 | 0 (omitted) 1996 | .2396167 3.494025 0.07 0.945 -6.608547 7.087781 1997 | .2233432 3.475928 0.06 0.949 -6.58935 7.036036 1998 | .2380076 3.470625 0.07 0.945 -6.564293 7.040308 1999 | .2342254 3.464307 0.07 0.946 -6.555692 7.024143 2000 | .2156315 3.443281 0.06 0.950 -6.533074 6.964337 2001 | .2237264 3.443018 0.06 0.948 -6.524465 6.971918 2002 | .220687 3.438196 0.06 0.949 -6.518053 6.959427 2003 | .2381433 3.447062 0.07 0.945 -6.517974 6.994261 2004 | .2287464 3.43517 0.07 0.947 -6.504062 6.961555 2005 | .2268003 3.425714 0.07 0.947 -6.487477 6.941077 2006 | .2278115 3.416657 0.07 0.947 -6.468712 6.924335 2007 | .230407 3.404762 0.07 0.946 -6.442803 6.903617 2008 | .2193702 3.393857 0.06 0.948 -6.432467 6.871207 2009 | .1918743 3.389242 0.06 0.955 -6.450919 6.834667 2010 | .1940647 3.390875 0.06 0.954 -6.451928 6.840058 2011 | .1777202 3.375464 0.05 0.958 -6.438068 6.793508 2012 | .1593265 3.361654 0.05 0.962 -6.429395 6.748048 2013 | .1416116 3.346188 0.04 0.966 -6.416796 6.700019 2014 | .124906 3.32236 0.04 0.970 -6.386799 6.636611 2015 | .1539902 3.330146 0.05 0.963 -6.372977 6.680957 2016 | 0 (omitted) | _cons | 0 (omitted)------------------------------------------------------------------------------Instruments for first differences equation Standard D.(lngdp lnes lntechnology 1992b.year 1993.year 1994.year 1995.year 1996.year 1997.year 1998.year 1999.year 2000.year 2001.year 2002.year 2003.year 2004.year 2005.year 2006.year 2007.year 2008.year 2009.year 2010.year 2011.year 2012.year 2013.year 2014.year 2015.year 2016.year) GMM-type (missing=0, separate instruments for each period unless collapsed) L(1/2).(lnopen lnei) collapsed L(1/2).lnco22 collapsedInstruments for levels equation Standard lngdp lnes lntechnology 1992b.year 1993.year 1994.year 1995.year 1996.year 1997.year 1998.year 1999.year 2000.year 2001.year 2002.year 2003.year 2004.year 2005.year 2006.year 2007.year 2008.year 2009.year 2010.year 2011.year 2012.year 2013.year 2014.year 2015.year 2016.year _cons GMM-type (missing=0, separate instruments for each period unless collapsed) D.(lnopen lnei) collapsed D.lnco22 collapsed------------------------------------------------------------------------------Arellano-Bond test for AR(1) in first differences: z = -0.61 Pr > z = 0.540Arellano-Bond test for AR(2) in first differences: z = -0.54 Pr > z = 0.591------------------------------------------------------------------------------Sargan test of overid. restrictions: chi2(0) =1187.85 Prob > chi2 = . (Not robust, but not weakened by many instruments.)Hansen test of overid. restrictions: chi2(0) = 5.54 Prob > chi2 = . (Robust, but weakened by many instruments.)
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