Frequencies for first 10,000 rows of hydro variable in natl1999 dataset :

Hydrocephal |
         us |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |          5        0.05        0.05
          2 |      9,995       99.95      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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