Frequencies for first 10,000 rows of ultrasnd variable in nhamcsed2005 dataset :

 ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
         no |      9,703       97.03       97.03
        yes |        297        2.97      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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