However, we didn't get a GAI score. We read that if there are multiple subset scores in the 19's and 18's that the tester should refer to extended norms. Our son had such scores but I don't think that we got that. All we got was a one pager handwritten. Can one use the subset scores to calculate the GAI? Should we have received more from the tester?
The GAI may or may not be relevant. It is only supposed to be reported when the VCI and PRI indices are significantly higher than the WMI or PSI indices. So, if you aren't seeing a profile where the full scale IQ (FSIQ) number is pulled way down by either the WMI or PSI or both, the GAI really isn't something that you'd need or which should be given in the report.
In regard to extended norms, yes they can be considered when you've got scores of 18s or 19s, but they may or may not increase the overall IQ number. It depends on the raw scores that fed into those scaled scores of 18 and 19. If you have the raw scores, we can help you figure out if extended norms apply for your child. If you have the scaled scores and a discrepancy btwn VCI/PRI and WMI/PSI, we can also help you figure a GAI.
You can also do that yourself if you google WISC technical report #4 (that one will help you figure a GAI) and WISC technical report #7 (that one will help you figure the extended norms and whether they apply).