Frequencies for first 10,000 rows of hydro variable in natl1993 dataset :

Hydrocephal |
         us |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |          4        0.04        0.04
          2 |      9,996       99.96      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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