问题已经得到解决,主要是样本量过小造成的。
- sysuse auto.dta,clear
- reg mpg turn gear_ratio ,robust
- estimate store eq_ols
- ivregress 2sls mpg gear_ratio (turn = weight length headroom),robust
- estimate store eq_2sls
- ivregress gmm mpg gear_ratio (turn = weight length headroom),robust
- estimate store eq_gmm
- ivregress liml mpg gear_ratio (turn = weight length headroom),robust
- estimate store eq_liml
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(1) (2) (3) (4)
eq_ols eq_2sls eq_gmm eq_liml
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turn -0.733*** -1.246*** -1.209*** -1.255***
(0.120) (0.197) (0.188) (0.199)
gear_ratio 3.033 -0.315 0.130 -0.369
(1.533) (1.863) (1.755) (1.871)
_cons 41.218*** 71.665*** 68.892*** 72.160***
(8.572) (12.687) (12.060) (12.782)
-------------------------------------------------------------------------------
N 74 74 74 74
-------------------------------------------------------------------------------
Standard errors in parentheses
* p<0.05, ** p<0.01, *** p<0.001