Frequencies for first 10,000 rows of ob_ecvs variable in natlterr2015 dataset :

 Successful |
   External |
   Cephalic |
  Version Y |
        Yes |      Freq.     Percent        Cum.
------------+-----------------------------------
            |      1,078       10.78       10.78
          N |      8,805       88.05       98.83
          U |        110        1.10       99.93
          Y |          7        0.07      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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