Frequencies for first 10,000 rows of hydro variable in natlterr1996 dataset :

Hydrocephal |
         us |      Freq.     Percent        Cum.
------------+-----------------------------------
          2 |      9,998       99.98       99.98
          9 |          2        0.02      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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