The thing to be careful of here is that the ITBS does not work like the MAPS. A standard score of 250, say, is not the same for a 3rd grader as it is for a 4th grader or a 5th grader. You really cannot compare across grade levels on the ITBS. The NCE, SS, and GE are all relative to how other kids would have tested or did test on that one grade level test only.
That is true to some extent, and I would certainly do further (out of level) testing before proposing anything like formal subject acceleration in a school setting, but a standard score of 250 is a standard score of 250 and it is very likely that the child would obtain that same SS if given the out of level test.
I don't think that it is on the ITBS for the reason mon gave above. A SS of 250 on a 3rd grade version of the ITBS is not the same as a SS of 250 on the 4th or 5th grade version. The GE and SS both refer to how children in the norming sample scored when given the 3rd grade version, which differs from other grade versions.
On some other tests like NWEA MAPs, the rit scores do compare across grade levels b/c the test is identical regardless of your grade or age. The student just moves up to harder questions as s/he answers questions correctly.
Edit to add: In looking at how the ITBS is scored further, I want to clarify. While standard scores apparently can be compared across grade levels to some extent (i.e. a SS of 244 would be the 50th percentile for the reading total for an 8th grader and, presumably, a much higher percentile for the reading total for a 3rd grader), they are still based on different tests. The 8th grade test, for instance, gives much harder questions than the 3rd grade test. So, if a 3rd grader got a 244 (answered correctly very difficult questions for a 3rd grader) on the 3rd grade test, I don't believe that it stands to reason that the same 3rd grader, if given the 8th grade test, would answer as well as the average (50th percentile) 8th grader, achieving the same SS of 244 when answering much harder questions.
