图没显示出来。我是这样的，求各位看看怎么解决，谢谢！
. xtbalance,range(2002 2011)

(36 observations deleted due to out of range)

. xtset stkcd year
       panel variable:  stkcd (strongly balanced)
        time variable:  year, 2002 to 2011
                delta:  1 unit

. xtunitroot llc roe
Levin-Lin-Chiu test requires strongly balanced data
r(498);

lz是怎么解决的，碰到同样的问题~~