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

     Failed |
   External |
   Cephalic |
  Version R |      Freq.     Percent        Cum.
------------+-----------------------------------
            |      3,422       34.22       34.22
          N |      6,571       65.71       99.93
          U |          1        0.01       99.94
          Y |          6        0.06      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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