Frequencies for first 10,000 rows of ob_ecvs variable in natl2017 dataset :

 Successful |
   External |
   Cephalic |
  Version Y |
        Yes |      Freq.     Percent        Cum.
------------+-----------------------------------
          N |      9,882       98.82       98.82
          U |         77        0.77       99.59
          Y |         41        0.41      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 19 Sep 2018