I used the processing speed subtests of the WISC-IV to confirm an unexpected outcome on the processing speed subtests of the WJ-III, and the resulting index score differed by 39 points. No changes in testing environment or subject behavior. The tests were administered a week apart. I followed standard administration procedures for both instruments, and had a colleague double check the scoring. In case an error in timing had affected the WJ-III results, I double-tripled checked timing during the WISC - IV, and those results were even more discrepant than the unexpected WJ-III results. Any ideas what else could contribute to a difference of this magnitude, or how it's even possible?