Frequencies for first 10,000 rows of refer variable in namcs2012 dataset :

   Was patient |
  referred for |
        visit? |      Freq.     Percent        Cum.
---------------+-----------------------------------
         Blank |        181        1.81        1.81
       Unknown |        720        7.20        9.01
Not applicable |      3,513       35.13       44.14
           Yes |      1,941       19.41       63.55
            No |      3,645       36.45      100.00
---------------+-----------------------------------
         Total |     10,000      100.00

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