11.6 Using your EAEF data set, investigate whether there is evidence that selection bias affects the least squares estimate of the returns to college education. Define COLLYEAR = S – 12 if S > 12, 0 otherwise, and LGEARNCL = LGEARN if COLLYEAR > 0, missing otherwise. Use the heckman procedure to regress LGEARNCL on COLLYEAR, MALE, ETHBLACK, and ETHHISP, with SM, SF, and SIBLINGS being used to determine whether the respondent attended college. Note that ASVABC has (deliberately) been excluded from both parts of the model. Then repeat the analysis (1) adding ASVABC to the probit part of the model (2) to both parts. Comment on your findings.