Frequencies for first 10,000 rows of preterm variable in natl1991 dataset :

   Previous |
 preterm or |
small-for-g |
estational- |
 age infant |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |         53        0.53        0.53
          2 |      9,939       99.39       99.92
          8 |          1        0.01       99.93
          9 |          7        0.07      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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