Frequencies for first 10,000 rows of refer variable in nhamcsopd2011 dataset :

   Was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         Blank |        483        4.83        4.83
       Unknown |        932        9.32       14.15
Not applicable |      2,551       25.51       39.66
           Yes |      2,037       20.37       60.03
            No |      3,997       39.97      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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