Frequencies for first 10,000 rows of refer variable in nhamcsopd2004 dataset :

   was patient |
  referred for |
   this visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         blank |        478        4.78        4.78
           yes |      3,273       32.73       37.51
            no |      2,600       26.00       63.51
       unknown |        822        8.22       71.73
not applicable |      2,827       28.27      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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