Frequencies for first 10,000 rows of hydro variable in linkco1991us_num dataset :

Hydrocephal |
         us |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |         63        0.63        0.63
          2 |      9,664       96.64       97.27
          9 |        273        2.73      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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