请教：在STATA10中运行出现如下问题

命令
xtgls ye size2 ndts2 grow2 prof statectrol top5 brc2 gdp2 cpi, panel(het)

xttest2

显示
Correlation matrix of residuals is singular.
not possible with test

.命令 xtserial l L.size L.profit L.special L.stock L.first L.fifth L.asset L.gnp L.n
> ///
L.growth L.risk
L.whole L.core L.sub L.s L.d L.co
结果：time-series operators not allowed

命令：xtreg l L.size L.profit L.special L.stock L.first L.fifth L.asset L.gnp L.n ///

L.growth L.risk
L.whole L.core L.sub L.s L.d L.co, fe

xttest2

结果：__000002 not found