Frequencies for first 10,000 rows of ultras variable in natl2000 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      7,986       79.86       79.86
          2 |      2,013       20.13       99.99
          9 |          1        0.01      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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