一，非平衡面板用xtscc：
xtset idd year
       panel variable:  idd (unbalanced)
        time variable:  year, 2001 to 2007, but with gaps
                delta:  1 unit
. xtscc  logtfp  exportstate absorb  size area year klindustry  ,fe
year is not regularly spaced: there are contemporaneous gap(s) across all subjects in idd
r(101）.