This is what I found from the NAGC's position statement on the WISC IV:
" Despite these limitations, testers should consult the Extended Norms if a child achieves at least one scaled score of 19 or two of 18 and modify scaled, Composite, GAI and Full Scale IQ scores accordingly. These can be found in Technical Report #7, available at
http://www.pearsonassessments.com/N...C-8E4A114F7E1F/0/WISCIV_TechReport_7.pdf or through a search for “WISC-IV Extended Norms.” Extended Norms have been helpful in identifying some exceptionally and profoundly gifted students for much-needed support services, although others encounter insufficient item difficulty with no benefit from Extended Norms."
