如图，第二阶段的自变量为什么被omitted掉了，该怎么处理？并且加了固定效应后，工具变量就变得不显著了。


代码如下：
ivregress2 2sls Capacity Size Lev Staff SOE Age TFP Market ROA Tophold Top2_10 Gdp_last Gdp_p (Gdptarget=city_number)i.year i.industry, first
est restore first
outreg2 using ivnew1.doc,cttop(first stage)  tstat bdec(3) tdec(2) keep(Capacity city_number Gdptarget ) addtext(Control variables, YES,Year FE,YES,Industry FE,YES,City FE,NO) append
est restore second
outreg2 using ivnew1.doc,cttop(second stage)  tstat bdec(3) tdec(2) keep(Capacity city_number Gdptarget ) addtext(Control variables, YES,Year FE, YES,Industry FE,YES,City FE,NO) append