Frequencies for first 10,000 rows of ab_surf variable in natl2013 dataset :

 Surfactant |
        RFP |
631=f_ab_su |
     rfac R |      Freq.     Percent        Cum.
------------+-----------------------------------
          N |      9,665       96.65       96.65
          U |        310        3.10       99.75
          Y |         25        0.25      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 22 Dec 2015