Description of files: TWINS90.DAT This is an ASCII file that contains a public use file with selected variables from the 1990 twins data Ashenfelter and Krueger used. The variables are separated by columns. The input statement in PUBLIC.SAS lists the proper order of the variables; this can easily be adapted if you want to use a program other than SAS. PUBLIC.SAS This is a mainframe SAS program that reads in the data from TWINS90.DAT. This program labels the variables. It also runs regressions from selected Tables in Ashenfelter and Krueger (1993). PUBLIC.LOG This contains the SAS listing file of PUBLIC SAS. TWIN2.SAS This is a SAS program that created the data in TWINS90.DAT.