Frequencies for first 10,000 rows of stratm variable in nhamcsopd1997 dataset :

 psu marker |
   (masked) |      Freq.     Percent        Cum.
------------+-----------------------------------
        201 |      6,086       60.86       60.86
        202 |      2,806       28.06       88.92
        203 |        959        9.59       98.51
        204 |        149        1.49      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 28 Mar 2016