Frequencies for first 10,000 rows of refer variable in nhamcsopd2006 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         Blank |        497        4.97        4.97
           Yes |      2,892       28.92       33.89
            No |      2,796       27.96       61.85
       Unknown |      1,012       10.12       71.97
Not applicable |      2,803       28.03      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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