Frequencies for first 10,000 rows of rf_ppterm variable in natl2015 dataset :

   Previous |
    Preterm |
Birth Y Yes |      Freq.     Percent        Cum.
------------+-----------------------------------
          N |      9,452       94.52       94.52
          U |        108        1.08       95.60
          Y |        440        4.40      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 20 Oct 2016