Frequencies for first 10,000 rows of amnio variable in natl1990 dataset :

Amniocentes |
         is |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |        218        2.18        2.18
          2 |      9,738       97.38       99.56
          9 |         44        0.44      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 3 Jan 2016