- IQ (Intelligence Quotient)
- IQ is a standardized score that summarizes general cognitive ability relative to the population. Scores are set to a mean of 100 and a standard deviation of 15, so 100 is exactly average.
- Deviation IQ
- Deviation IQ is the modern way of computing IQ as 100 + 15 × z, comparing you against same-age peers rather than using a mental-age formula. It replaced the older ratio IQ and is what every major test uses today.
- Ratio IQ
- Ratio IQ is the historical method that divided a child's mental age by chronological age and multiplied by 100. It worked for children but broke down for adults, so it was replaced by the deviation IQ.
- General Intelligence (g)
- The g factor is the single common ability that underlies performance across virtually all cognitive tasks. Because mental tests tend to correlate positively, g is treated as the core of what IQ measures.
- Fluid Intelligence (Gf)
- Fluid intelligence is the ability to reason, spot patterns, and solve novel problems without relying on prior knowledge. It peaks in the early-to-mid 20s and gradually declines thereafter.
- Crystallized Intelligence (Gc)
- Crystallized intelligence is accumulated knowledge, vocabulary, and learned skills. Unlike fluid intelligence, it keeps rising well into your 60s as experience builds.
- Percentile
- A percentile tells you the percentage of people who score at or below your level. An IQ at the 98th percentile means you scored higher than 98% of the population.
- Standard Deviation (SD)
- Standard deviation measures how spread out scores are around the mean. On the Wechsler scale the IQ standard deviation is 15, so most people fall between 85 and 115.
- Normal Distribution (Bell Curve)
- The normal distribution is the symmetric bell-shaped curve that IQ scores follow, with most people clustered near the mean and few at the extremes. It lets any score be translated directly into a percentile.
- Mean (Average)
- The mean is the arithmetic average of all scores. For IQ the mean is fixed at 100 by definition, marking the center of the distribution.
- Standardization (Norming)
- Standardization is the process of administering a test to a large, representative sample to set the norms against which everyone else is scored. It is what makes 100 the true population average.
- Age Norming
- Age norming scores you against people of your own age group rather than against everyone. This keeps the average at 100 for every age, which is why raw fluid-reasoning ability can fall while your age-adjusted IQ stays stable.
- Reliability
- Reliability is how consistently a test produces the same result on repeat measurement. It is often reported as Cronbach's alpha, where good cognitive tests typically score 0.85 or higher.
- Validity
- Validity is whether a test actually measures what it claims to measure. A test can be highly reliable yet still lack validity if it predicts the wrong things.
- Raven's Progressive Matrices
- Raven's Progressive Matrices is a nonverbal test in which you complete a visual pattern by choosing the missing piece. It is one of the purest measures of fluid intelligence and the g factor because it relies on reasoning rather than language or learned knowledge.
- CHC Theory (Cattell–Horn–Carroll)
- CHC theory is the leading scientific model of cognitive abilities, organizing intelligence into a general g factor over broad abilities like fluid reasoning, comprehension-knowledge, and processing speed. Most modern IQ tests are built around its framework.
- Wechsler Scales (WAIS / WISC)
- The Wechsler scales are the clinical gold-standard IQ tests, with the WAIS for adults and the WISC for children. They use a standard deviation of 15 and are administered individually by trained professionals.
- Stanford–Binet
- The Stanford–Binet is one of the earliest standardized IQ tests and is still in use today. It traditionally uses a standard deviation of 16 rather than the Wechsler 15, so identical scores can differ slightly between the two.
- Mensa
- Mensa is the best-known high-IQ society, open to anyone who scores at or above the 98th percentile, the top 2%. That threshold corresponds to roughly an IQ of 130 on a standard-deviation-15 scale.
- Flynn Effect
- The Flynn effect is the steady rise in average IQ test performance over the 20th century, on the order of about 3 points per decade. Because tests are periodically re-normed, the population mean is reset back to 100.
- Gifted
- Gifted typically refers to an IQ of about 130 or above, placing a person in roughly the top 2% of the population. It is the same threshold used for Mensa eligibility.
- Working Memory
- Working memory is the ability to hold and manipulate information in mind for a few seconds, such as keeping numbers in your head while calculating. It is closely linked to fluid reasoning and is a core component in modern IQ models.
- Processing Speed
- Processing speed is how quickly you can perform simple, routine cognitive tasks accurately. It is one of the broad abilities in CHC theory and contributes to overall IQ, especially on timed tests.
- Intellectual Disability
- Intellectual disability is defined by significantly below-average intelligence, an IQ around 70 or below, combined with limitations in everyday adaptive skills. Both the score and real-world functioning must be considered, not the number alone.