Frequencies for first 10,000 rows of refer variable in nhamcsopd2005 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         blank |        411        4.11        4.11
           yes |      3,030       30.30       34.41
            no |      2,404       24.04       58.45
       unknown |        776        7.76       66.21
not applicable |      3,379       33.79      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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