Frequencies for first 10,000 rows of nervous variable in natl1997 dataset :

      Other |
    central |
    nervous |
     system |
  anomalies |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |          1        0.01        0.01
          2 |      9,987       99.87       99.88
          9 |         12        0.12      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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