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

Amniocentes |
         is |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |        441        4.41        4.41
          2 |      9,348       93.48       97.89
          9 |        211        2.11      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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