Frequencies for first 10,000 rows of refer variable in nhamcsopd2002 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         Blank |        675        6.75        6.75
           Yes |      2,460       24.60       31.35
            No |      3,148       31.48       62.83
       Unknown |        714        7.14       69.97
Not applicable |      3,003       30.03      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 28 Mar 2016