Frequencies for first 10,000 rows of hydro variable in natl2002 dataset :

Hydrocephal |
         us |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |          2        0.02        0.02
          2 |      9,583       95.83       95.85
          9 |        415        4.15      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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