Intelligence, aptitude and achievement tests: Wechsler, Stanford-Binet, and the rest
Last reviewed September 2026 by Dr. Andrew Wichterman · Aligned to the current NCE blueprint. Valid for exams through June 30, 2027.
AI-narrated overview made with Google NotebookLM. The written lesson below is by Dr. Andrew Wichterman.
Achievement, aptitude and intelligence tests each ask a different question about a person. This lesson tells the story of the IQ from Binet to Wechsler, walks through the tests the exam names, and explains the theories of intelligence in a sentence each.
Try to answer each from memory first, even if you have to guess. Guessing before reading makes the answer stick; reveal only after you have tried.
Start here: three questions a test can ask
Tests of ability come in three flavors, and the difference is when they are looking. An achievement test asks what you have already learned: a final exam, a reading test. An aptitude test asks how well you are likely to do at something in the future: the SAT is supposed to predict college performance. An intelligence test asks about general mental ability, the broad capacity to reason, learn and solve problems, without tying it to a school subject. The exam mostly wants you to name a test from a description and to know a little history, so this lesson is a tour of the tests with the stories behind them.
A short history of the IQ
In 1905 in France, Alfred Binet and Théodore Simon built the first practical intelligence test. The government wanted a way to identify schoolchildren who needed extra help, and Binet's test did that by comparing a child's performance to what was typical for each age. A child who solved the problems most 8-year-olds could solve had a mental age of 8.
In 1916 Lewis Terman at Stanford adapted the test for the United States, creating the Stanford-Binet. Around the same time William Stern proposed dividing mental age by chronological (actual) age and multiplying by 100 to get a single number. That was the original IQ, the "intelligence quotient," and the word "quotient" is a leftover from that division. A 10-year-old with a mental age of 12 had an IQ of 120.
The ratio IQ had a problem: mental age stops rising in the late teens, so the formula breaks down for adults. David Wechsler solved it in 1939 by dropping the division altogether and instead comparing each person to others of the same age, then placing them on a normal curve with a mean of 100 and a standard deviation of 15. That is the deviation IQ used by every major test today. An IQ of 115 no longer means anything about mental age; it means one standard deviation above the average for your age group.
The Wechsler tests: pick by age
The Wechsler scales are the most widely used intelligence tests in the world, and the exam tests them by age. There are three, and their names tell you who they are for.
| Test | Stands for | Ages | Current edition |
|---|---|---|---|
| WPPSI | Wechsler Preschool and Primary Scale of Intelligence | About 2½ to 7 | WPPSI-IV |
| WISC | Wechsler Intelligence Scale for Children | 6 to 16 | WISC-V |
| WAIS | Wechsler Adult Intelligence Scale | 16 to 90 | WAIS-5 (2024); the WAIS-IV is still widely used |
The Wechsler tests give a Full Scale IQ plus several index scores: verbal comprehension, visual-spatial, fluid reasoning, working memory and processing speed. Older items and older editions talk about a "Verbal IQ" and a "Performance IQ"; the modern editions replaced that split with the index scores. All Wechsler scores use the same scale: mean 100, SD 15, so a score of 130 is two SDs above average, the 98th percentile.
The other ability tests you should recognize
| Test | What it is | Why the exam mentions it |
|---|---|---|
| Stanford-Binet 5 | Intelligence test for ages 2 to adult; the modern descendant of Binet's original | The other major IQ test; historically used SD 16 |
| Kaufman (KABC, KAIT) | Intelligence tests built on a theory of how children process information; the KABC is often used with culturally diverse children | Alternative to Wechsler; "less culturally loaded" |
| Raven's Progressive Matrices | Nonverbal reasoning: pick the pattern that completes a grid of shapes | The example of a "culture-fair" test; no language needed |
| Peabody Picture Vocabulary Test | Point to the picture that matches a spoken word; no reading or speaking required | Receptive vocabulary; useful when a child cannot read or write |
| Woodcock-Johnson | Battery of achievement and cognitive tests, kindergarten through adult | Common in schools for learning-disability evaluation |
| WIAT | Wechsler Individual Achievement Test: reading, math, writing | Often paired with the WISC to compare ability against achievement |
| Otis-Lennon School Ability Test | Group-administered school ability test | Example of a group (not individual) intelligence test |
| Army Alpha and Beta | World War I group tests; Alpha for readers, Beta (pictures) for those who could not read English | The first mass intelligence testing; Beta was the first nonverbal group test |
| ASVAB | Armed Services Vocational Aptitude Battery | The most widely used aptitude test; used for military placement and career counseling |
| DAT | Differential Aptitude Tests | Measures several separate aptitudes (verbal, numerical, mechanical…) for career guidance |
Individual versus group tests
An individual test is given one-on-one by a trained examiner who reads the instructions, watches the person work and scores their responses. The Wechsler and Stanford-Binet are individual tests. A group test is given to many people at once with a booklet or a screen, like the SAT or the Otis-Lennon. Individual tests are richer (the examiner sees how the person approaches problems) but expensive and slow; group tests are efficient but tell you less about any one person. A counselor giving a WAIS needs specific training; a group aptitude test can be administered by a proctor.
What is intelligence, anyway? The theories
The exam names several people who argued about what intelligence is. Each gets one idea.
| Name | One idea | Everyday version |
|---|---|---|
| Spearman | g, general intelligence | One underlying ability shows up in everything you do; people who are good at one mental task tend to be good at others |
| Thurstone | Seven primary mental abilities | No single g; instead separate abilities like verbal comprehension, number, spatial, memory |
| Cattell (with Horn) | Fluid vs. crystallized intelligence | Fluid = raw reasoning power, peaks young. Crystallized = accumulated knowledge, keeps growing with age |
| Guilford | Structure of intellect; convergent vs. divergent thinking | Convergent = finding the one right answer. Divergent = generating many possible answers (creativity) |
| Gardner | Multiple intelligences | Eight or more separate kinds: musical, bodily, interpersonal… not just school smarts |
| Sternberg | Triarchic theory | Three parts: analytical (book smarts), creative, practical (street smarts) |
| Vernon | Hierarchical model | g on top, broad group factors below, specific skills at the bottom; a compromise between Spearman and Thurstone |
The fluid/crystallized distinction shows up in aging items too: crystallized intelligence (vocabulary, general knowledge) holds up well into old age, while fluid intelligence (speed, novel problem solving) declines. That is why an older adult can be slower on a new puzzle yet know more words than a 20-year-old.
Two things that complicate IQ scores
The Flynn effect is the finding that average raw scores on IQ tests rose across the 20th century, about three points per decade. Because of it, tests have to be re-normed every so often; a person taking a 30-year-old test against 30-year-old norms will look smarter than they are.
Test bias is the concern that a test measures different things for different groups, usually because the content assumes a particular culture or language. A vocabulary item about sailing is easier for children who grew up near boats. This is why "culture-fair" tests like Raven's use shapes instead of words, and why the exam expects you to consider the norm group and the client's background before interpreting a score.
How the exam asks about this
Age picks the test. "A 5-year-old is referred for an intelligence evaluation." WPPSI. "A 40-year-old." WAIS. "A 10-year-old." WISC. Watch the overlap: a 16-year-old could take either the WISC or the WAIS.
Purpose picks the type. "Predict how a student will do in a training program" is aptitude. "Measure what students learned in the course" is achievement. "Broad reasoning ability" is intelligence.
Name the person. "Who proposed a general factor, g?" Spearman. "Who distinguished fluid and crystallized?" Cattell. "Who built the first practical intelligence test?" Binet. "Who developed the deviation IQ?" Wechsler.
Interpret a score. "A client's WAIS Full Scale IQ is 70." Two SDs below the mean, the 2nd percentile; the traditional cutoff for considering intellectual disability, though a diagnosis also requires adaptive-functioning deficits.
Without scrolling up, write everything you remember from this lesson. Then check yourself against the key terms. Recalling the whole lesson once, right after reading it, does more than reading it twice.
Ten more questions from the whole domain, with an explanation after each answer.
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