连老师: 关于上次问您的现金-现金流敏感性,使用工具变量解决托宾q的内生性问题,我还有两个问题想问您,
1、当我对所有样本用ivreg回归后,使用ivendog命令检验变量的内生性问题,结果是在1%的水平上拒绝原假设,说明存在内生性问题。但是当对样本分组,仅对其中某个子样本进行检验时,无法拒绝原假设,模型不存在内生性问题。我觉得很困惑,是不是样本改变后内生性问题就不存在了,这种情况该在实证文章中该如何解释?
2、利用ivreg2 命令检验工具模型设定的合理性,使用不同选项时获得了不同的统计量
当利用ivreg2 不附加任何选项时,结果如下:
Underidentification test (Anderson canon. corr. LM statistic): 1067.591
Chi-sq(4) P-val = 0.0000
Weak identification test (Cragg-Donald Wald F statistic): 342.221
Stock-Yogo weak ID test critical values: 5% maximal IV relative bias 16.85
10% maximal IV relative bias 10.27
20% maximal IV relative bias 6.71
30% maximal IV relative bias 5.34
10% maximal IV size 24.58
15% maximal IV size 13.96
20% maximal IV size 10.26
25% maximal IV size 8.31
Source: Stock-Yogo (2005). Reproduced by permission.
Sargan statistic (overidentification test of all instruments): 12.126
Chi-sq(3) P-val = 0.0070
当时用ivreg2附加gmm2s robust选项时汇报的是
Underidentification test (Kleibergen-Paap rk LM statistic): 191.460
Chi-sq(4) P-val = 0.0000
Weak identification test (Cragg-Donald Wald F statistic): 342.221
(Kleibergen-Paap rk Wald F statistic): 60.460
Stock-Yogo weak ID test critical values: 5% maximal IV relative bias 16.85
10% maximal IV relative bias 10.27
20% maximal IV relative bias 6.71
30% maximal IV relative bias 5.34
10% maximal IV size 24.58
15% maximal IV size 13.96
20% maximal IV size 10.26
25% maximal IV size 8.31
Source: Stock-Yogo (2005). Reproduced by permission.
NB: Critical values are for Cragg-Donald F statistic and i.i.d. errors.
Hansen J statistic (overidentification test of all instruments): 7.246
Chi-sq(3) P-val = 0.0645
但是文献中经常汇报的是Aderson LR和Hansen J统计量,如果写命令来同时获得这两个统计量呢?
期待您的解答,非常感谢!