Frequencies for first 10,000 rows of prov1990 variable in pos1991 dataset :

        540 |
   antibody |
   detect + |
      other |      Freq.     Percent        Cum.
------------+-----------------------------------
            |      8,536       85.36       85.36
          N |      1,145       11.45       96.81
          Y |        319        3.19      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 26 Jan 2016