Frequencies for first 10,000 rows of ultras variable in natlterr1998 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      3,570       35.70       35.70
          2 |      6,428       64.28       99.98
          9 |          2        0.02      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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