I'm going to pipe in here as well, because our experience contradicts the study cited above and other anecdotes that suggest RIAS correlates, but tends to be higher than the WISC-IV. My DD was tested with the RIAS at school just after she turned 6.
Her VIX on the RIAS was only 118, but her NIX was over 140, which was pretty much the opposite of what I would have expected. Because of the discrepancy, to double check the NIX score, and find out whether I was in denial about possible DYS qualifications I choose to have her privately tested shortly thereafter.
Her scores on the WISC-IV were 138 and 137 with comparable processing speed and slightly lower working memory, for an FSIQ of 143 and and GAI of 144. So the NIX was close, but the VIX and the full scale scores were way off.
I will admit that I honestly have my doubts regarding the school's administration of the RIAS, so maybe we are the outlier for a completely unrelated reason.
Just a different perspective.
