Frequencies for first 10,000 rows of ultras variable in natlterr2001 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      4,107       41.07       41.07
          2 |      5,869       58.69       99.76
          8 |         20        0.20       99.96
          9 |          4        0.04      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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