面板数据固定效应模型要如何进行分组回归?
gen dummy=0
replace dummy=1 if ln_research>-5.239343
xtreg total_quality_e EG2 EG2_square total_quality_elag ln_capital_intensity ln_scale ln_age ln_subsidy ln_research tfp_opva ownership i.year i.provincecode i.industry2,fe if dummy=1
这样做显示显示option if not allowed,应该如何解决?