Frequencies for first 10,000 rows of ultras variable in natl2001 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      8,143       81.43       81.43
          2 |      1,855       18.55       99.98
          8 |          1        0.01       99.99
          9 |          1        0.01      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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