OK, I've been curious about this for a while. Why does the GAI exclude both WMI and PSI? Are they frequently similar scores?
Because neither of them loads as heavily on general intelligence (g) as the VCI and PRI do, and there is some support in the factor analysis for a two-factor solution consisting of VCI/PRI and WMI/PSI. (See Dumont-Willis scoring.) In addition, validation studies support the use of the GAI as it is currently defined to discriminate the gifted population from NTs. Sometimes they are similar (especially in gifted/non2e), where a lower CPI results from two comparable contributing indices, usually in the 110s, in which case you can appropriately discuss the cognitive proficiency cluster as a unit. In 2e, it is fairly common to find any combination of one or both lower, generally average or below.
Speculatively, a non 2e, relatively low PSI might reflect a deliberate working style, with high attention to detail, or perfectionistic tendencies, or prioritizing accuracy over speed, all of which are essentially varying degrees of the same thing, a pattern familiar to most persons working with gifted kids.
(Other explanations include inattention/poor sustained attention, impulsivity (if the number of items completed is within expectations, but poor accuracy lowers the standard/scaled score), fine motor deficits, vision or visual tracking issues, emotional interference, such as from anxiety or depression, or psychomotor retardation as a side effect of some medications, especially the psychotropics used to treat anx/dep.)
