Frequencies for first 10,000 rows of ultrasnd variable in namcs1998 dataset :

 ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |      9,787       97.87       97.87
          1 |        213        2.13      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 28 Mar 2016