Frequencies for first 10,000 rows of prov1600 variable in pos1992 dataset :

    patient |
   dialysis |
   training |
       cert |      Freq.     Percent        Cum.
------------+-----------------------------------
            |      9,744       97.44       97.44
          A |        191        1.91       99.35
          B |         61        0.61       99.96
          C |          1        0.01       99.97
          E |          3        0.03      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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