命令： madfuller share,lags(2(2)8)
结果如下：
Multivariate Augmented Dickey-Fuller test for share
with 22 observations on 18 cross-sectional units
-----------------------------------------------------------
       Obs    Lags        MADF      Approx 5% CV
-----------------------------------------------------------
        20       2    6391.644         38.897

Covariance matrix of errors is singular
r(506);
出现了Covariance matrix of errors is singular 错误，为什么不能检验出4阶 8阶的呢？？