Frequencies for first 10,000 rows of ultrasnd variable in namcs1997 dataset :

 ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |      9,758       97.58       97.58
          1 |        242        2.42      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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