Frequencies for first 10,000 rows of refer variable in nhamcsopd2008 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         blank |        394        3.94        3.94
       unknown |      1,346       13.46       17.40
not applicable |      2,598       25.98       43.38
           yes |      2,625       26.25       69.63
            no |      3,037       30.37      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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