Frequencies for first 10,000 rows of ob_ecvs variable in natl2016 dataset :

 Successful |
   External |
   Cephalic |
  Version Y |
        Yes |      Freq.     Percent        Cum.
------------+-----------------------------------
          N |      9,910       99.10       99.10
          U |         52        0.52       99.62
          Y |         38        0.38      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 4 Oct 2017