Frequencies for first 10,000 rows of ab_surf variable in natlterr2011 dataset :

 Surfactant |
        RFP |
631=f_ab_su |
     rfac R |      Freq.     Percent        Cum.
------------+-----------------------------------
            |      2,536       25.36       25.36
          N |      6,302       63.02       88.38
          U |      1,094       10.94       99.32
          Y |         68        0.68      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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