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

   Cervical |
 Cerclage R |      Freq.     Percent        Cum.
------------+-----------------------------------
            |      3,422       34.22       34.22
          N |      6,538       65.38       99.60
          U |          1        0.01       99.61
          Y |         39        0.39      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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