FSIQ includes measures of working memory and processing speed, neither of which is highly correlated with general intelligence, "g", but both of which can affect real-world functioning in significant ways. GAI only includes reasoning ability measures, which are the best measure of "g". According to the technical guidance, in general, when there is a significant difference between FSIQ and GAI, the GAI should be taken as the more accurate representation of intelligence, provided that there is not also a significant difference between PRI and VCI. (If the index scores - VCI, PRI, WMI, and PSI, - are significantly discrepant from each other, they obviously can not be combined into a valid composite - not that this stops some people from trying...)
