Frequencies for first 10,000 rows of op_ecvs variable in natlterr2011 dataset :

 Successful |
   External |
   Cephalic |
Version RFP |
603=f_ob_su |
       cc R |      Freq.     Percent        Cum.
------------+-----------------------------------
            |      2,536       25.36       25.36
          N |      6,304       63.04       88.40
          U |      1,142       11.42       99.82
          Y |         18        0.18      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 22 Dec 2015