另外麻烦解释一下这个问题:
. xtreg lnvio shall i.year,fe r
must specify panelvar; use xtset
r(459);
. xtset lnvio shall i.year,fe r
factor-variable and time-series operators not allowed
r(101);
. xi:xtreg lnvio shall i.year,fe r
i.year _Iyear_77-99 (naturally coded; _Iyear_77 omitted)
must specify panelvar; use xtset
r(459);
. xi:xtset lnvio shall i.year,fe r
i.year _Iyear_77-99 (naturally coded; _Iyear_77 omitted)
too many variables specified
r(103);
感谢!