Frequencies for first 10,000 rows of ultras variable in linkco1991us_num dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      4,629       46.29       46.29
          2 |      5,159       51.59       97.88
          8 |          1        0.01       97.89
          9 |        211        2.11      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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