Frequencies for first 10,000 rows of refer variable in nhamcsopd2010 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         blank |        577        5.77        5.77
       unknown |        976        9.76       15.53
not applicable |      3,267       32.67       48.20
           yes |      2,409       24.09       72.29
            no |      2,771       27.71      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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