Frequencies for first 10,000 rows of ultras variable in natl1995 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      7,622       76.22       76.22
          2 |      2,351       23.51       99.73
          9 |         27        0.27      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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