According to the statistical summary NUMATS put out, they use scores from more than one year in order to have a larger sample size. The pool included in the summary this year was:
Explore:
3rd grade: 1,098
4th grade: 2,965
5th grade: 4,545
6th grade: 3,625
6th grade: ACT: 3,040 SAT: 2,482
7th grade: ACT: 7,617 SAT: 7,651
8th grade: ACT: 6,956 SAT: 10,554
9th grade: ACT: 3,760 SAT: 444
I don't have the pool size for the Belin-Blank search, but I can tell you that I ran my dd9's scores through their interactive site and then had the NUMATS summary by mail. Her percentile's were higher in Belin-Blank by a significant amount(within the 4th grade pool), which leads me to believe that the NUMATS pool is more competitive.
71.1N vs 83bb; 87.6N vs.95bb; 100n vs. 99bb; 74N vs. 81bb; 93N vs. 97bb.
I think the NUMATS scores are probably a more accurate reflection of where she is at right now, much as I would like to claim the higher Belin-Blank rankings
