求帮助呀,在用xtabond2命令时,加入的GMM工具变量为一解释变量的一期到三期滞后,为什么只有一阶差分残差的一,二阶序列相关的P值呢
命令如下:xi:xtabond2 fs di12 lpgdp lpgdp2 Coefficiet urb indpser cpi i.country i.year,gmm(di12,lag(1 3)) iv(cpi i.year ) small
回归结果中部分
Arellano-Bond test for AR(1) in first differences: z = 4.18 Pr > z = 0.000
Arellano-Bond test for AR(2) in first differences: z = 0.07 Pr > z = 0.944
Sargan test of overid. restrictions: chi2(64) = 49.73 Prob > chi2 = 0.905
(Not robust, but not weakened by many instruments.)