I pulled those numbers off of the NWEA RIT Scale Norms (MAP test) report. So a fourth grader at the 94th percentile will have a GE of 8.9 (or 8th grade in the spring) and an 8th grader with the same RIT score (who took an equivalent test) will be at the 50th percentile and have the same GE of 8.9. The ITBS data that I have (in the form of multiple score reports spanning grades K-12) seems to correlate with the MAP data.
So in this case, presumably the fourth grader with the 8.9 GE would score in the 50th percentile on the 8th grade ITBS--assuming that he could finish the test.
The key here is that average 8th graders have not mastered 8th grade material, they've mastered 4th grade material.
Of course, the 8th grader has more life experience and probably does know more than the 4th grader about many things. Just not about the things being tested.
