use"C:\Users\acer\Desktop\stata\nlswork.dta", clear
xtset idcode year
global xlist 'grade age ttl_exp tenure not_smsa south race'
gen time=(year>=77)&!missing(year)
gen treated=(idcode>2000)&!missing(idcode)
gen did=time*treated
reg ln_wage did time treated $xlist


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