Frequencies for first 10,000 rows of ultras variable in natlterr1999 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      3,642       36.42       36.42
          2 |      6,355       63.55       99.97
          9 |          3        0.03      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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