Frequencies for first 10,000 rows of ab_surf variable in natl2017 dataset :

 Surfactant |
      Y Yes |      Freq.     Percent        Cum.
------------+-----------------------------------
          N |      9,891       98.91       98.91
          U |         72        0.72       99.63
          Y |         37        0.37      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 19 Sep 2018