Frequencies for first 10,000 rows of uop_amnio variable in natlterr2005 dataset :

Amniocentes |
       is U |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |         42        0.42        0.42
          2 |      4,430       44.30       44.72
          9 |      1,783       17.83       62.55
          . |      3,745       37.45      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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