identifier
81 to 87
annual wage, 1987 $
years of schooling
years work experience
=1 if in manufacturing
=1 if black
log(wage)
=1 if year == 81
=1 if in labor force
exper^2
lwage[_n-1] if year > 81
inlf[_n-1] if year > 81
inlf[_n+1] if year < 87