代码如下：
search permute  //先安装此命令

permute did beta = _b[did] se = _se[did] df = e(df_r), seed(10000) ///
reps(500) saving("安慰剂检验.dta"): ///
    reghdfe y did $c ,a(id year)

use "安慰剂检验.dta" , clear
gen t_value = beta / se
gen p_value = 2 * ttail(df, abs(beta/se))
replace beta = beta

#delimit ;
                twoway (kdensity beta, yaxis(1) lc(black) ) (scatter p_value beta, yaxis(2) m(Oh) mc(black))  ,
                 xline(0, lc(black*0.5) lp(solid))
                 yline(0.05, lc(black*0.5) lp(dash) axis(2))
                 xtitle("Estimator", size(*0.8)) xlabel(, format(%4.1f) labsize(small))
                 ytitle("Density", size(*0.8)) ylabel(, nogrid format(%4.1f) labsize(small))
                 ytitle("P Value", size(*0.8) axis(2)) ///
                 ylabel(, nogrid format(%4.1f) labsize(small) axis(2))
                 legend(ring(2) order(1 "Estimator" 2 "P Value") pos(6) rows(1) )
                 graphregion(color(white)) scheme(s1mono) ;
#delimit cr

graph export "安检验.png", width(1000) replace