Psychometrics & cognitive-science glossary
25 plain-English definitions of the key terms used across IQ research, online cognitive testing, and modern Cattell-Horn-Carroll psychometric theory.
Cattell-Horn-Carroll model (CHC)
The dominant contemporary framework for organizing cognitive ability research. Three strata: g at the top, ten broad abilities at the second level (Gf, Gc, Gsm, Glr, Gv, Ga, Gs, Gt, Grw, Gq), and dozens of narrow abiliti…
Full definitionCeiling effect
The phenomenon where test-takers above a certain ability level all score at the maximum possible score, losing the ability to discriminate among them. A test with a low ceiling cannot meaningfully measure exceptional abi…
Full definitionCrystallized intelligence (Gc)
The breadth and depth of knowledge accumulated through education, reading, and life experience. Measured by vocabulary tests, general-information tests, and reading comprehension. Rises through middle age and is largely …
Full definitionFactor analysis
The statistical technique used to identify latent variables (factors) that account for shared variance across observed measures. The mathematical foundation of all modern intelligence research. Applied to a battery of co…
Full definitionFloor effect
The phenomenon where test-takers below a certain ability level all score at the minimum possible score, losing the ability to discriminate among them. A test with a high floor cannot meaningfully measure low cognitive ab…
Full definitionFluid intelligence (Gf)
The ability to reason and solve novel problems with minimal prior knowledge required. Measured most directly by matrix-reasoning tasks, number-series items, and analogical-reasoning items with abstract content. Peaks in …
Full definitionFlynn effect
The systematic rise in average IQ scores across the 20th century — about 3 IQ points per decade in industrialized countries. Named for political scientist James Flynn. Largest on fluid-reasoning measures; smallest or abs…
Full definitiong (general intelligence factor)
The general factor extracted by factor analysis of cognitive test batteries, accounting for the positive manifold of correlations across diverse cognitive tests. First proposed by Charles Spearman in 1904. Modern interpr…
Full definitionGifted
A designation typically used in school placement contexts for students scoring at or above the 95th to 98th percentile on a cognitive ability test. The most common single threshold is IQ 130. Gifted designation usually q…
Full definitionICAR (International Cognitive Ability Resource)
A public-domain catalog of validated cognitive ability items, developed by William Revelle and colleagues at Northwestern University. Released items, scoring keys, and norming data freely to qualified researchers. Includ…
Full definitionItem difficulty
The proportion of a reference sample that answers a particular item correctly. Easy items (proportion correct > 0.7) discriminate poorly among above-average test-takers; hard items (proportion correct < 0.3) discriminate…
Full definitionItem discrimination
The degree to which an item separates high-ability from low-ability test-takers. High-discrimination items are answered correctly substantially more often by high-ability than by low-ability test-takers; low-discriminati…
Full definitionMensa
An international high-IQ society, founded in 1946, requiring scores at or above the 98th percentile on a battery of approved cognitive tests. The qualifying threshold is roughly IQ 130 on the WAIS-IV and Stanford-Binet 5…
Full definitionNormal distribution
The bell-shaped probability distribution that describes the distribution of IQ scores in the population, by construction. About 68% of scores fall within 1 SD of the mean (IQ 85 to 115), about 95% within 2 SD (IQ 70 to 1…
Full definitionOpen-Source Psychometrics Project
An online platform hosting dozens of public-domain personality and ability tests since 2011, including a 25-item Raven-style matrix test. Each completed test contributes anonymized data to a public dataset that can be do…
Full definitionPercentile rank
The proportion of a reference population scoring at or below a given score, expressed as a percentage. A percentile of 75 means the test-taker scored higher than 75% of the reference population. Equivalent to z-score and…
Full definitionProcessing speed (Gs)
The rate at which simple cognitive operations can be performed. Measured by digit-symbol coding, visual search, and simple reaction time. Correlates with general intelligence at r ≈ 0.5. Shows the steepest age-related de…
Full definitionRaven's Progressive Matrices
A nonverbal IQ test developed by John Raven in 1938, consisting of 60 multiple-choice items of increasing difficulty. Each item is a 3×3 (or 2×2) grid of geometric figures with one cell missing. Widely used as a measure …
Full definitionReliability
The consistency of a measurement instrument across repeated administrations or alternate forms. Expressed as a correlation coefficient between 0 and 1. Test-retest reliability for major adult IQ batteries is typically 0.…
Full definitionStandard error of measurement (SEM)
The expected variability in a measured score across repeated administrations of the same test, due to measurement error alone. The 95% confidence interval for a score is approximately ±2 SEM. Clinical adult IQ tests have…
Full definitionStanford-Binet 5 (SB5)
One of the two flagship clinical IQ tests used in English-speaking countries, alongside the Wechsler scales. Traces lineage to the 1916 Stanford revision of the Binet-Simon scale. The fifth edition (SB5), published in 20…
Full definitionValidity
The degree to which a test measures what it claims to measure. Multiple flavors: construct validity (does the test measure the underlying construct), criterion validity (does the test predict relevant outcomes), content …
Full definitionWAIS-IV (Wechsler Adult Intelligence Scale)
The most widely used clinical measure of adult cognitive ability in English-speaking countries. Published by Pearson in 2008. Administered one-on-one by a trained psychologist in 60 to 90 minutes. Yields a Full-Scale IQ …
Full definitionWorking memory
The cognitive system that holds and manipulates information over short timescales (seconds). Distinct from passive short-term memory. Modern models (Baddeley) divide working memory into a central executive, phonological …
Full definitionZ-score (standard score)
The number of standard deviations a score is above or below the mean of its reference distribution. Computed as z = (score - mean) / SD. A z of +1 corresponds to an IQ of 115 and the 84th percentile; a z of -1 correspond…
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