Frequencies for first 10,000 rows of refer variable in nhamcsopd2001 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         Blank |        844        8.44        8.44
           Yes |      2,295       22.95       31.39
            No |      3,270       32.70       64.09
       Unknown |      1,050       10.50       74.59
Not applicable |      2,541       25.41      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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