Frequencies for first 10,000 rows of refer variable in namcs2008 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         blank |        298        2.98        2.98
       unknown |        382        3.82        6.80
not applicable |      7,951       79.51       86.31
           yes |        449        4.49       90.80
            no |        920        9.20      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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