求助大神帮忙解答,面板数据做完随机效应回归后出现如图(或如下文)所示回归结果,其中sigma_u=0该如何解释~~~大谢~!
Random-effects GLS regression Number of obs = 674
Group variable: city1 Number of groups = 71
R-sq: within = 0.0359 Obs per group: min = 1
between = 0.0414 avg = 9.5
overall = 0.0380 max = 16