Frequencies for first 10,000 rows of ultras variable in natl1990 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      6,510       65.10       65.10
          2 |      3,446       34.46       99.56
          9 |         44        0.44      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 3 Jan 2016