WAIS index scores, explained

The Wechsler Adult Intelligence Scale gives one overall number, the Full Scale IQ (FSIQ), and several index scores underneath it. Each index combines two or three subtests that measure a related skill, so it is more reliable than any single subtest and more specific than the FSIQ.

The fourth edition (WAIS-IV, 2008) has four indexes. The fifth (WAIS-5, 2024) splits perceptual reasoning in two, giving five. Your report will say which edition you took; the rest of this page works for both.

Type your index scores below to see each one as a percentile and in the WAIS-IV wording, and how far apart your highest and lowest scores are. It does the arithmetic a report leaves to you. It does not replace the psychologist's reading, which uses your history, the subtests and the publisher's tables.

Every index uses the same scale as the FSIQ, explained in standard deviation in IQ. 100 is the middle of your age group, 115 is one standard deviation above (84th percentile) and 85 one below (16th). The report normally gives a percentile and a 90% or 95% confidence interval next to each score; the interval is the honest version of the number. An index of 112 with an interval of 105 to 118 overlaps an index of 104 with an interval of 97 to 111, so the two have not been shown to differ. The IQ confidence interval calculator shows how wide the band is for a given reliability.

The FSIQ is an average of very different tasks. When your indexes are close, the average describes you well. When they are far apart, say a VCI of 128 and a PSI of 88, the FSIQ lands somewhere in the middle and describes neither strength nor weakness.

A widely taught rule of thumb from Flanagan and Kaufman's interpretive method is to subtract the lowest index from the highest. Under 23 points (1.5 standard deviations), treat the FSIQ as a fair summary. At 23 or more, describe the person by their indexes instead, and look at whether the General Ability Index is a better overall number. Other authors, including Sattler, argue there is no hard cut-off; the 23 points is a convention, not a law.

Big gaps are also more common than people expect. Research on healthy adults (Binder, Iverson and Brooks, 2009) shows that a few low scores and noticeable scatter are normal in people with no condition at all. A gap is a reason to read the indexes, not a diagnosis.^[1]

The General Ability Index (GAI) is built from the verbal and reasoning subtests only, leaving out working memory and processing speed. It answers "how well does this person reason, setting speed and short-term memory aside?" On the WAIS-IV it uses the VCI and PRI subtests.

The Cognitive Proficiency Index (CPI) is the other half: working memory and processing speed together, the efficiency with which you handle information.

When the GAI is clearly higher than the CPI, reasoning is outrunning efficiency. Psychologists see this pattern in many people with attention difficulties, with fatigue or after a brain injury, but also in plenty of people with none of these, so a GAI-CPI gap points to questions, not answers. The checker above shows the average of your reasoning indexes and your speed and memory indexes as a rough guide to that pattern; the real GAI and CPI need the publisher's tables.

These are tendencies, not rules, and a single index never explains a person on its own.

The editions are close but not interchangeable. In a study of 186 adults who took both, the WAIS-IV gave Full Scale IQs about 1.9 points higher on average (Winter, Trudel and Kaufman, 2024), the Flynn effect at work on the older norms. The WAIS-5 VSI and FRI also have no direct WAIS-IV equivalent, so compare them with the old PRI only loosely. For more on the test itself and how to book one, see the WAIS test guide.^[2]