谢谢老师指教,确实发现记错了,r2没有明显变化,但是加入聚类后我的DID模型,政策对于light(夜间灯光)的影响就不显著了,请问还有办法挽救吗。
我做了下伍德里奇检验,P<0.001,F值30左右。
. xtreg light did time treated,fe
Fixed-effects (within) regression Number of obs = 1033
Group variable: ID Number of groups = 150
R-sq: within = 0.0182 Obs per group: min = 5
between = 0.0037 avg = 6.9
overall = 0.0086 max = 7
F(3,880) = 5.43
corr(u_i, Xb) = 0.0043 Prob > F = 0.0011
------------------------------------------------------------------------------
light | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
did | .6953325 .3342992 2.08 0.038 .0392156 1.351449
time | .2756154 .1054371 2.61 0.009 .0686779 .4825529
treated | -.4670529 .8331767 -0.56 0.575 -2.102298 1.168192
_cons | .8324569 .1179802 7.06 0.000 .6009015 1.064012
-------------+----------------------------------------------------------------
sigma_u | 1.533692
sigma_e | 1.453613
rho | .52678718 (fraction of variance due to u_i)
------------------------------------------------------------------------------
F test that all u_i=0: F(149, 880) = 7.26 Prob > F = 0.0000
. xtreg light did time treated,fe vce(cluster)
invalid vce(cluster) option
r(198);
. xtreg light did time treated,fe vce(cluster ID)
Fixed-effects (within) regression Number of obs = 1033
Group variable: ID Number of groups = 150
R-sq: within = 0.0182 Obs per group: min = 5
between = 0.0037 avg = 6.9
overall = 0.0086 max = 7
F(3,149) = 2.31
corr(u_i, Xb) = 0.0043 Prob > F = 0.0784
(Std. Err. adjusted for 150 clusters in ID)
------------------------------------------------------------------------------
| Robust
light | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
did | .6953325 .7582237 0.92 0.361 -.8029275 2.193592
time | .2756154 .1272484 2.17 0.032 .0241709 .5270599
treated | -.4670529 .6837773 -0.68 0.496 -1.818206 .8841
_cons | .8324569 .094411 8.82 0.000 .6458996 1.019014
-------------+----------------------------------------------------------------
sigma_u | 1.533692
sigma_e | 1.453613
rho | .52678718 (fraction of variance due to u_i)
------------------------------------------------------------------------------
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