Frequencies for first 10,000 rows of amnio variable in natlterr2002 dataset :

Amniocentes |
         is |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |        138        1.38        1.38
          2 |      9,846       98.46       99.84
          8 |         14        0.14       99.98
          9 |          2        0.02      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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