Frequencies for first 10,000 rows of ultras variable in natl1993 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      7,436       74.36       74.36
          2 |      2,534       25.34       99.70
          9 |         30        0.30      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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