Frequencies for first 10,000 rows of op_ecvs variable in linkpe2009psden dataset :

 Successful |
   External |
   Cephalic |
  Version R |      Freq.     Percent        Cum.
------------+-----------------------------------
            |      3,422       34.22       34.22
          N |      6,564       65.64       99.86
          U |          1        0.01       99.87
          Y |         13        0.13      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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