Frequencies for first 10,000 rows of refer variable in nhamcsopd2009 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         blank |        742        7.42        7.42
       unknown |      1,469       14.69       22.11
not applicable |      2,245       22.45       44.56
           yes |      2,768       27.68       72.24
            no |      2,776       27.76      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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