Frequencies for first 10,000 rows of op_ecvs variable in natl2013 dataset :

 Successful |
   External |
   Cephalic |
Version RFP |
603=f_ob_su |
       cc R |      Freq.     Percent        Cum.
------------+-----------------------------------
          N |      9,767       97.67       97.67
          U |        213        2.13       99.80
          Y |         20        0.20      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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