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Determining FSIQ from GAI score

By Inderbir Kaur Sandhu, Ph.D

Q: Is there a way to determine FSIQ from my son's GAI score on the WISC IV? (He was given an individual test at 6.3 years)

His results were:

GAI 160
VCI: 148
I: 18
S: 18
(V): 17
C: 18

PRI: 155
BD: 16
PCn: 22
MR: 19
(PCm): 13

He was also given the Stanford Binet IV:

Quantitative Reasoning Index: 161
Quantitative SS: 150
Number Series SS: 160.

A: Unfortunately, this is not possible. The GAI (General Ability Index) score only considers the VCI (verbal comprehension index) and the PRI (perceptual reasoning index). For the Full Scale IQ, all 10 subtests are required to calculate the FSIQ and the four Index scores are included in the standard battery.

The GAI (VCI + PRI) is used in place of the FSIQ. When the FSIQ is not interpretable, it has to be determined whether an abbreviated GAI may be used to describe overall intellectual ability based on the following question:

  • Is the size of the standard score difference between the VCI and PRI less than 1.5 standard deviations (< 23 points)?

If the answer is YES, then the GAI may be calculated and interpreted as a reliable and valid estimate of a child's overall intellectual ability.

If the answer is NO, then the variation in the Indexes that compose the GAI is too great (>23 points) for the purpose of summarising overall ability in a single score (i.e., GAI).

In your son's case, it is a YES since the GAI score was calculated. The following are the reason a tester may decide to use the GAI score and not the FSIQ: There may be:

  1. A significant and unusual discrepancy exists between VCI and WMI

  2. A significant and unusual discrepancy exists between PRI and PSI

  3. A significant and unusual discrepancy exists between WMI and PSI, or

  4. A significant and unusual subtest discrepancy between subtests within WMI and/or PSI.

Hope the above helps. Please see the tester for further interpretation. Best of luck!


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