Frequencies for first 10,000 rows of hydro variable in natl1997 dataset :

Hydrocephal |
         us |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |          6        0.06        0.06
          2 |      9,982       99.82       99.88
          9 |         12        0.12      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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