Frequencies for first 10,000 rows of ultras variable in natl1994 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      7,312       73.12       73.12
          2 |      2,671       26.71       99.83
          9 |         17        0.17      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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