Frequencies for first 10,000 rows of refer variable in nhamcsopd2003 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         blank |        295        2.95        2.95
           yes |      2,701       27.01       29.96
            no |      2,680       26.80       56.76
       unknown |      1,156       11.56       68.32
not applicable |      3,168       31.68      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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