Frequencies for first 10,000 rows of refer variable in nhamcsopd2007 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         blank |        548        5.48        5.48
       unknown |      1,205       12.05       17.53
not applicable |      3,063       30.63       48.16
           yes |      3,152       31.52       79.68
            no |      2,032       20.32      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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