Pearson sent me the technical report for the extended norms earlier this week - if anyone would like to PM me their email address I'd be happy to forward it.
I'd actually asked them about the compressed qualifying scores for LOG in WPPSI-III that are on the Hoagies site, but haven't managed to get myself clear about this yet. They may have mistaken me for someone who really understood much about this!
Excerpt:
These norms are used to further
differentiate highly gifted children (FSIQ or
GAI above 150) from gifted children (FSIQ
or GAI 130�150). The extended norms are
useful when a child�s score is the
maximum (ceiling) on two or more
subtests (e.g., obtains scaled scores of 18
or 19 points). These norms extend the
WISC�IV upper scaled score range to 28
points for subtests and 210 points for
composites. It is important to note that the
extended norms are not useful for most
children. Among the 2,200 cases in the
WISC�IV standardization sample, only
one child obtained a GAI score of 151 and
none obtained an FSIQ score of 150 or
higher. According to the bell curve, out of
20,000,000 same-age peers, only one
child would be expected to obtain a FSIQ
of 180 or higher.
