Hi JamD,
It has been a while since I have looked at the extended norms, so I'm very rusty on the subject. But here is what I remember.
The use of the extended norms is determined by the raw score of a subtest. It is not dependent on how many 19s or 18s you have. I think psychologists just use the number of 19s or 18s to gauge whether it is worth the effort to plug the raw scores into the extended norms table or not. Sometime a subtest score of 19 will be a soft ceiling, and will not translate to a different score on the extended norms. And sometime a 19 will be a hard ceiling and the score will change substantially with the extended norms. The more 19s a child has, the higher the probability that one of those 19 will be a hard ceiling.
My son had three 19s and an 18 on the WISC-IV, but only one of them had a raw score that was in the range of the extended norms. When the raw score was corrected with the use of extended norms table, the scaled score jumped from a 19 to a 22 and his PRI score changed by a full 10 points, from 145 to a 155.
So I would recommend looking at the raw score for the Similarities. The 19 scaled score may be a soft ceiling or a hard ceiling 19. You really won't know until you look up the score on the Extended Norms table. It doesn't take much time or effort on your part, and it would give you some peace of mind to know.
The other thing to keep in mind is that, while we treat the WISC-IV as the "end all, be all" for IQ testing, it is really just a snap shot of your child on one particular day. I don't know how many kids end up taking the WISC-IV twice over several years, but I think the scores have some movement or fluidity to them. My DS12 scored in the DYS level both times, at a 7 year old and as a 10 year old, but the scores changed quite a bit. One year the PRI was 10 points higher than the VCI, and then the next time they flipped. The VCI was 10 points higher than the PRI. So I am a firm believer in the view that any test is just a small piece of information for a large and complex puzzle. Maybe that is because my child is VERY complex, with significant 2Es: ADHD, sensory issues, and a large helping of asynchronous development. I would guess that for a more balanced child, the scores would be more stable.
Hope this helps?
