Frequencies for first 10,000 rows of ultrasnd variable in nhamcsed2013 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
         No |      9,527       95.27       95.27
        Yes |        473        4.73      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 7 Nov 2016