gen did1=before3*_treated (政策前三年 T-3)
gen did2=before2*_treated
gen did3=before1*_treated (政策前一年 T-1)
gen did4= current*_treated
gen did5= after1*_treated
gen did6= after2*_treated
gen did7= after3*_treated
xtreg Y did1 did2 did4 did5 did6 did7 _treated 时间虚拟变量 x1 x2 x3, fe