Frequencies for first 10,000 rows of amnio variable in natl1998 dataset :

Amniocentes |
         is |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |        150        1.50        1.50
          2 |      9,848       98.48       99.98
          9 |          2        0.02      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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