Intelligence and Its Measurement (SACE Stage 2 Psychology)

Syllabus dot point

“Explain competing definitions of intelligence and evaluate how it is measured, including IQ testing and issues of reliability, validity and bias”

8 min readUpdated 2 Jun 2026

Quick answer

Intelligence is broadly the capacity to learn, reason and adapt to the environment, but psychologists disagree on whether it is a single general ability or many distinct abilities. It is measured with standardised IQ tests that produce a score around a mean of 100, and these tests must be evaluated for reliability, validity and cultural bias.

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What this dot point is asking
Defining intelligence
Theories of intelligence
Measuring intelligence
Evaluating the measures
Exam-style questions

What this dot point is asking

You need to outline competing definitions of intelligence, describe how it is measured, and evaluate the strengths and limitations of those measures.

Defining intelligence

There is no single agreed definition, but most include the ability to learn from experience, reason, solve problems and adapt to new situations. Theories differ in whether intelligence is one thing or many.

Theories of intelligence

General intelligence (g): Charles Spearman argued that a single underlying factor, g, influences performance across all mental tasks, supported by the finding that scores on different tests tend to correlate.
Multiple intelligences: Howard Gardner proposed that there are several relatively independent intelligences (for example linguistic, logical-mathematical, spatial, musical, bodily-kinaesthetic, interpersonal, intrapersonal and naturalistic), arguing that a single score misses important abilities.
Triarchic theory: Robert Sternberg proposed three aspects: analytical (problem-solving), creative (dealing with novelty) and practical (everyday adaptive) intelligence.
Fluid and crystallised intelligence: Cattell distinguished fluid intelligence (reasoning and solving novel problems, which tends to decline with age) from crystallised intelligence (accumulated knowledge and skills, which tends to increase or hold).

These theories sit on a spectrum from "intelligence is one thing" (Spearman) to "intelligence is many things" (Gardner). Sternberg and Cattell occupy a middle ground, accepting some general capacity while insisting that distinct kinds of ability exist. SACE answers earn marks for placing a named theory on this spectrum and explaining its evidence, not just listing names.

Measuring intelligence

Intelligence is measured using standardised tests. Modern tests such as the Wechsler scales (WAIS for adults, WISC for children) and the Stanford-Binet report an IQ score. Historically, IQ was calculated as mental age divided by chronological age multiplied by 100; modern tests instead use a deviation IQ that compares a person's score to others of the same age.

Scores are standardised so that the population mean is 100 with a standard deviation of about 15, producing a normal (bell-shaped) distribution. Because the curve is symmetrical, about $68\%$ of people score within one standard deviation of the mean (between 85 and 115), and about $95\%$ score within two standard deviations (between 70 and 130). Scores below 70 or above 130 are each held by only about $2.5\%$ of the population.

A test is standardised when it is administered and scored in a uniform way and compared against norms from a large, representative sample. Tests are periodically re-normed because average raw performance has risen over time (the Flynn effect), so the mean must be reset to 100.

Evaluating the measures

Good psychological tests must be reliable, valid and free from bias:

Reliability - the test produces consistent results. Test-retest reliability checks whether the same person scores similarly on different occasions.
Validity - the test measures what it claims to measure. Predictive validity asks whether scores predict relevant outcomes such as academic performance.
Cultural bias - items that assume knowledge, language or experiences common to one cultural group can disadvantage others, lowering validity. Culture-fair tests try to reduce reliance on language and culturally specific knowledge.

Other limitations: a single score may oversimplify a complex set of abilities, tests can be affected by motivation, anxiety and test-taking practice, and scores can be misused to label people.

The normal distribution of IQ

IQ scores are scaled to a mean of 100 and a standard deviation of about 15, forming a bell-shaped normal distribution. This means most people cluster near the average and very high or very low scores are rare.

Reading an IQ score against the normal curve

State the parameters

A standardised IQ test has a mean ( $\mu$ ) of 100 and a standard deviation ( $\sigma$ ) of 15. A student scores 130.

Convert the score to standard deviations

The score is $130 - 100 = 30$ points above the mean. Dividing by the standard deviation, $30 / 15 = 2$ , so the student is exactly two standard deviations above the mean.

Interpret using the empirical rule

About $95\%$ of scores fall within two standard deviations of the mean, leaving $5\%$ in the two tails combined and about $2.5\%$ above $+2\sigma$ . So a score of 130 is roughly the 98th percentile: only about 2 to 3 students in 100 score this high.

Add the evaluation point

A precise-looking percentile still rests on the test being reliable, valid and free of cultural bias for this student, so the number should be reported with those limits in mind rather than as a fixed label.

Common errors

Saying IQ measures how clever someone is overall: IQ measures performance on particular tasks under test conditions; many psychologists argue it captures only part of intelligence.
Confusing reliability and validity: Reliability is consistency; validity is accuracy. A test can be reliable (consistent) without being valid (measuring the right thing).
Ignoring cultural bias: A test that relies on culturally specific language or knowledge can produce misleadingly low scores for some groups, which is a validity problem.

Exam-style questions

Questions in the style of SACE Board exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

SACE 2022-style6 marks

A researcher administers the same standardised IQ test to a sample of 200 Year 12 students twice, six weeks apart. The two sets of scores correlate at

r = 0.91

. (a) Identify the type of reliability being measured and state what the result shows. (b) Explain why high reliability does not guarantee that the test is a valid measure of intelligence. (c) Suggest one feature of the test that could reduce its validity for students from a non-English-speaking background.

Show worked answer →

This is a research/data item marked on the investigation and evaluation criteria. Work through the three parts in order.

(a) Type and meaning (2 marks): The design (same test, same people, two occasions) measures test-retest reliability. An $r = 0.91$ is a strong positive correlation, showing the test produces highly consistent scores across occasions, so it is reliable.
(b) Reliability is not validity (2 marks): Reliability is consistency; validity is whether the test measures what it claims to (intelligence). A test can give the same wrong answer every time: it could consistently measure test-taking skill, reading speed or familiarity with the format rather than underlying intelligence. So a high test-retest correlation tells you the scores are stable, not that they are accurate measures of the construct.
(c) Threat to validity (2 marks): Any item that relies on English vocabulary, idiom or culturally specific knowledge (for example a verbal-reasoning question using uncommon English words) would disadvantage students whose first language is not English, lowering their scores for reasons unrelated to intelligence. This is cultural/linguistic bias, which reduces validity for that group.

SACE 2021-style8 marks

Psychologists disagree about whether intelligence is a single general ability or a set of distinct abilities. Evaluate this debate, referring to at least two named theories, and discuss one implication of the debate for how intelligence should be measured.

Show worked answer →

This is an extended-response item marked on knowledge, application and evaluation. Use a define-compare-evaluate-implication structure.

Define the debate: The core question is whether intelligence is one underlying capacity (the "single factor" view) or several relatively independent abilities (the "multiple abilities" view).
Theory 1, single factor: Spearman's $g$ proposes one general factor underlying performance on all mental tasks. Evidence: scores on different cognitive tests tend to correlate positively (the "positive manifold"), implying a shared factor.
Theory 2, multiple abilities: Gardner's multiple intelligences (linguistic, logical-mathematical, spatial, musical, bodily-kinaesthetic, interpersonal, intrapersonal, naturalistic) argues a single score ignores real, valued abilities. Sternberg's triarchic theory (analytical, creative, practical) makes a similar point.
Evaluate: The $g$ view is parsimonious and predicts outcomes such as academic and job performance, but it may reduce a rich construct to one number. The multiple-abilities view is intuitively appealing and inclusive, but some proposed "intelligences" are hard to measure reliably and critics argue they are talents rather than intelligences.
Implication for measurement: If $g$ is real, a single broad IQ score is defensible. If intelligence is multiple, a single score is misleading and assessment should sample several domains. A top answer concludes with a reasoned position, for example that a general factor plus profile of specific abilities best fits the evidence.

Practise this

Sources & how we know this

Stage 2 Psychology subject outline (2025 onwards) — SACE Board
Stage 2 Psychology: external assessment — SACE Board