Frequencies for first 10,000 rows of ultrasnd variable in nhamcsed2007 dataset :

 ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
         no |      9,598       95.98       95.98
        yes |        402        4.02      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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