Frequencies for first 10,000 rows of ob_ecvs variable in natl2015 dataset :

 Successful |
   External |
   Cephalic |
  Version Y |
        Yes |      Freq.     Percent        Cum.
------------+-----------------------------------
          N |      9,851       98.51       98.51
          U |        117        1.17       99.68
          Y |         32        0.32      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 20 Oct 2016