Using all 12 months of the 2011 basic CPS datafiles downloaded from NBER, I run a frequency of the variable PRCHLD for the population ages 16 to 24 and get 3,666,839.

Using all 12 months of the 2011 basic CPS datafiles from IPUMS, I run a frequency of the variable NCHILD for the population ages 16 to 24 and get 2,863,036.

Incidentally, I do get the same number of total population age 16 to 24.

I am not sure why I get different frequencies, and think that it must have something to do with the variable construction. Any thoughts? Thank you!