Psychology research · Reference
What does an IQ test measure?
An IQ test is a standardised instrument that estimates aspects of cognitive ability relative to a reference population; understanding what it measures means examining its standardisation, reliability, validity, and well-documented limitations.
What IQ tests measure
IQ tests sample performance across tasks such as verbal reasoning, working memory, processing speed, and spatial or fluid reasoning, and combine them into composite scores. The result is interpreted relative to a representative norm group: a score of 100 is set as the average, and other scores express how far a person sits above or below that mean. The tests aim to estimate aspects of what psychologists call general cognitive ability, often summarised as the statistical factor g, rather than knowledge of any single subject.
Standardisation, reliability and validity
As psychometric instruments, IQ tests are evaluated on standardisation (administration and scoring under fixed conditions against representative norms), reliability (consistency of scores across items, time, and raters), and validity (whether the test measures what it claims).
Well-constructed IQ tests show high test-retest reliability and internal consistency, and their scores correlate with outcomes such as academic and certain job performance. These correlations are real but moderate, and they describe group-level relationships rather than fixed individual destinies.
The Flynn effect and limitations
A key finding in the measurement of intelligence is the Flynn effect: across the twentieth century, raw IQ-test performance rose substantially from generation to generation, requiring periodic re-norming of tests. This shows that scores reflect more than a fixed biological quantity and are sensitive to environment, education, and test familiarity. Other limitations include potential cultural bias in item content, the fact that any single number compresses a multidimensional ability, and the risk of over-interpreting scores beyond what their validity evidence supports.
A measurement-science perspective
Treating IQ as a measurement problem, rather than a verdict, keeps the focus on the quality of the instrument. Responsible use means citing the norms and version used, reporting confidence intervals around scores, and being explicit about what the test predicts and how strongly. These are the same psychometric principles that apply to any standardised assessment, and they are why the research community discusses IQ in terms of reliability, validity, and limitations rather than as a definitive label of a person's worth.
Key facts
At a glance
- Type: standardised psychometric test of cognitive ability
- Score scaling: population mean set to 100
- Estimates: general cognitive ability (the g factor)
- Evaluated on: standardisation, reliability, validity
- Flynn effect: average raw scores rose over the 20th century
- Limitations: cultural bias, single-number compression, re-norming
Common questions
FAQ
What does an IQ test actually measure?+
It estimates aspects of general cognitive ability — such as reasoning, working memory, and processing speed — relative to a norm group, with the average set to 100. It does not measure knowledge of a specific subject or a person's overall worth.
What is the Flynn effect?+
The Flynn effect is the observed rise in average raw IQ-test performance across generations during the twentieth century. It shows that scores are influenced by environment and education, and it requires tests to be periodically re-normed.
Are IQ tests reliable and valid?+
Well-constructed IQ tests show high reliability and predict certain outcomes such as academic performance at moderate levels. However, validity has limits: scores can reflect cultural and educational factors, and a single number cannot capture the full range of cognitive abilities.
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