Frequencies for first 10,000 rows of cstratm variable in nhamcsopd2002 dataset :

  clustered |
psu stratum |
     marker |
   (masked) |      Freq.     Percent        Cum.
------------+-----------------------------------
   2.01e+07 |      6,743       67.43       67.43
   2.02e+07 |      2,133       21.33       88.76
   2.03e+07 |      1,124       11.24      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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