Free explainer4 min read
Intelligence testing
Understand the model, recognise it in a stem, separate the look-alikes, then apply it.
Start with the mental model
This concept covers how IQ (intelligence quotient) is measured, and the key facts examiners test about the main tests and their scoring. For example, imagine assessing a 4-year-old versus a 40-year-old, the exact same test battery would not suit both. In practice, knowing which named test applies to which age group is exactly what these questions test.
Watch it explainedAn IQ score measures no quantity of anything. It reports a position in a crowd.
Visual explanationUse the diagram to rebuild the concept from memory.
01Core model
The Wechsler tests are age-specific: WPPSI for pre-schoolers, WISC for school-age children, and WAIS for adults. They give a Full-Scale IQ plus index scores (verbal comprehension, reasoning, working memory, processing speed), scaled to a mean of 100 with an SDStandard deviation, a measure of how spread out a set of numbers is.Read more → (standard deviation, a measure of spread) of 15. Raven's Progressive Matrices is a mostly non-verbal pattern-completion test of abstract reasoning, less dependent on language, though no test is fully culture-free. For example, imagine testing a patient who speaks little English using Raven's Progressive Matrices instead of a verbal Wechsler subtest. In practice, this means choosing a less language-dependent test can give a fairer estimate of ability when language is a barrier. The WAIS also separates 'hold' subtests, such as picture completion, which stay relatively resistant to age-related decline, from 'don't hold' subtests, such as processing-speed tasks, which decline more with age. WAIS-IV digit span has three parts: forward span (repeat the digits in the order given), backward span (reverse the order), and sequencing (recall the digits in ascending numerical order). The WAIS-IV General Ability Index (GAI) combines only the verbal comprehension and perceptual reasoning index scores, deliberately excluding working memory and processing speed, so it can stay higher than the Full-Scale IQ in a patient whose attention or speed is impaired.
02Recognise it in an SBA
• A psychologist requires a standardised Wechsler intelligence test for a 9-year-old child. Which instrument is designed for this age group?
• A researcher wants a largely non-verbal measure of abstract reasoning that reduces dependence on acquired vocabulary and language. Which test is most suitable?
• Successive generations obtain higher raw scores on established intelligence tests, requiring periodic renorming so that the population mean remains stable. What is this phenomenon called?
03Separate the look-alikes
| IQ | standardised to a mean of 100 and standard deviation of 15, so about 68% fall between 85 and 115, unlike raw scores. |
|---|---|
| Intellectual disability | an IQ below 70 with impaired adaptive function and childhood onset, rather than a low IQ alone. |
| Crystallised intelligence | acquired knowledge relatively preserved with age, whereas fluid intelligence declines. |
| Flynn effect | a secular rise in population IQ over decades, unlike an individual's stable relative score. |
| General Ability Index (GAI) vs Full-Scale IQ | GAI drops working memory and processing speed, so it reads higher than the Full-Scale IQ when those two domains are the ones impaired, unlike the Full-Scale IQ, which factors them in. |
| Picture completion (a 'hold' subtest) | relatively resistant to age-related decline, unlike processing-speed 'don't hold' subtests. |
How the exam thinksRead the question the way the examiner wrote it.
What people get wrong here
• The Wechsler Adult Intelligence Scale is designed for adolescents and adults rather than this child.
• The National Adult Reading Test estimates premorbid intelligence and is not a child IQ battery.
• The preschool and primary scale is intended for younger children below the usual WISC age range.
• The National Adult Reading Test depends on pronunciation and acquired verbal knowledge.
• A verbal comprehension index deliberately measures language-mediated knowledge and reasoning.