请问,
## robust significance test, cluster by group
## (robust vs. serial correlation)
coeftest(zz, vcov=vcovHC)
这个是不是考虑了时间序列相关结果呀?
## idem, cluster by time period
## (robust vs. cross-sectional correlation)
coeftest(zz, vcov=function(x) vcovHC(x, method="arellano",
type="HC1", cluster="group"))
这个是不是考虑了截面相关的回归结果呀?