The RIAS is intended as a quick assessment, cheap to deliver, since a certified tester can perform it in 20-40 minutes, as opposed to several hours with an instrument like the WISC. It generally tracks well with other instruments, but it has been known to err in two ways:
- Underestimates, due to low cut-off thresholds. A subject needs only to miss two consecutive questions for that section to be ended and scored.
- Overestimates among the gifted population, possibly due to the same phenomenon.
Therefore, it's possible that the VIX was an underestimate, the NIX was an overestimate, and the CIX is right about on target, if that makes any sense.
I would not rely on the RIAS as a diagnostic tool. If you're concerned that there might be some kind of learning disability or vision issue (and yes, a 1.5 SD spread between VIX and NIX is highly correlated with those), then I would suggest you start with a more thorough evaluation.
