Derivation
Created when each data source is appended together, e.g.:
use ident intdate marital using ${newstata}\ahs_ahs1, clear
gen ques="AHS"
gen slf=1
append using ${newstata}\cen_individ, keep(ident intdate marital spoucur spoupast self informtid)
replace ques="CEN" if ques==""
etc.