. xthreg2 dcc size L.roa gdp, rx(lnin) qx(lnin) thnum(1) bs(300) trim(0.05) grid(100) r
There exist time-invariant individual(s) (maybe only one obs):  dcc size L.roa gdp lnin
                 <istmt>:  3499  thestm2() not found
为什吗会出现这种情况，求救