大家好 我想要用双向固定效应模型做实证,但是采用命令xtreg y x i.quarter,fe r时,F()和Prob>F均没有数值,如下:
Fixed-effects (within) regression Number of obs = 352
Group variable: id Number of groups = 16
R-sq: Obs per group:
within = 0.4457 min = 22
between = . avg = 22.0
overall = 0.0925 max = 22
F(15,15) = .
corr(u_i, Xb) = -0.0000 Prob > F = .
(Std. Err. adjusted for 16 clusters in id)
而当把命令中的稳健标准误r删掉时,就又显示了,如下:
Fixed-effects (within) regression Number of obs = 352
Group variable: id Number of groups = 16
R-sq: Obs per group:
within = 0.4457 min = 22
between = . avg = 22.0
overall = 0.0925 max = 22
F(21,315) = 12.06
corr(u_i, Xb) = -0.0000 Prob > F = 0.0000
请问这是如何导致的?此时还能够再fe后加r吗?