What IQ
Measures.

1 The General Factor: g

At the core of IQ testing is the concept of general intelligence (g), discovered by British psychologist Charles Spearman in 1904. He observed a remarkable phenomenon: people who perform well on one type of cognitive task tend to perform well on most others. This "positive manifold" suggests an underlying general ability that influences performance across all cognitive domains.

Spearman used factor analysis to identify this common factor—g—that accounts for the correlations among diverse cognitive tests. In statistical terms, g is the first principal component extracted from a battery of cognitive tasks. It typically accounts for 40-50% of the variance in individual differences on cognitive tests.

Modern IQ tests like ACIS are designed to maximize their loading on g—the common variance shared across all cognitive tasks. ACIS achieves a remarkable .99 correlation with g, meaning it captures nearly all of this general cognitive capacity. This is accomplished through carefully selected subtests that tap into diverse cognitive abilities while all loading substantially on the general factor.

What exactly is g? Researchers have proposed various interpretations: processing speed, working memory capacity, neural efficiency, or the ability to form abstract representations. Most likely, g reflects a combination of these factors working together as a system for efficient information processing.

2 Fluid vs. Crystallized Intelligence

In the 1940s, Raymond Cattell made a fundamental distinction that transformed our understanding of intelligence:

This distinction has profound implications. A person's fluid and crystallized abilities may differ substantially. An older adult might have exceptional vocabulary and general knowledge (Gc) but struggle with novel abstract puzzles (Gf). A young person might show remarkable abstract reasoning but limited factual knowledge due to less life experience.

ACIS measures both types extensively—fluid reasoning through Matrix Reasoning, Figure Series, and similar subtests; crystallized intelligence through Vocabulary, Information, and verbal reasoning tasks. This dual measurement provides a complete picture of cognitive capacity.

Interestingly, fluid and crystallized intelligence interact dynamically: higher Gf facilitates faster learning, which builds greater Gc over time. This is called the "investment theory" of intelligence—fluid ability is "invested" in learning, producing crystallized knowledge.

3 The Major Cognitive Domains

Beyond g, modern IQ tests measure several distinct cognitive domains, each contributing unique information:

• Verbal Comprehension: Language knowledge, vocabulary depth, verbal reasoning, and the ability to understand and express ideas through words. This domain is highly correlated with academic success, particularly in humanities and social sciences. It reflects both innate language aptitude and environmental exposure to language.
• Visual-Spatial Processing: Mental rotation, spatial visualization, perceiving relationships among objects, and constructing mental models of physical space. Critical for STEM fields, architecture, surgery, athletics, and any activity requiring manipulation of objects or navigation through space.
• Fluid Reasoning: Solving novel problems, identifying patterns, drawing inferences, and thinking logically when prior knowledge doesn't help. The "purest" measure of reasoning ability, least influenced by education, most heritable, and most predictive of learning potential.
• Working Memory: The ability to hold information in conscious awareness and manipulate it—like a mental workspace. Crucial for complex cognition: following multi-step instructions, mental arithmetic, reading comprehension, and any task requiring simultaneous processing of multiple pieces of information.
• Processing Speed: The speed and efficiency of simple cognitive operations under time pressure. While seemingly simple, processing speed underlies more complex abilities and is sensitive to brain health, aging, and neurological conditions.
• Quantitative Reasoning: Mathematical knowledge, numerical facility, and the ability to reason with numbers and quantities. Combines crystallized knowledge (learned math facts) with fluid reasoning (solving novel quantitative problems).

Each domain provides unique clinical and educational information. A person might have strong verbal abilities but weaker spatial skills—valuable information for career planning and identifying optimal learning strategies.

4 What IQ Does NOT Measure

Understanding the boundaries of IQ testing is as important as knowing what it measures. IQ tests are powerful tools, but they don't capture everything we might call "intelligence" in everyday usage:

• Emotional Intelligence: The ability to perceive, understand, and manage emotions in yourself and others. Some research suggests EI is relatively independent of IQ and predicts certain outcomes (like relationship quality) that IQ does not.
• Creativity: While moderately correlated with IQ (correlations around .20-.40), creative thinking involves divergent processes and ideational fluency that standard IQ tests don't fully capture. You need a minimum IQ threshold for creativity, but beyond that, more IQ doesn't guarantee more creativity.
• Practical Intelligence: Sometimes called "street smarts"—tacit knowledge for solving real-world problems. Knowing how to navigate a bureaucracy, read social situations, or handle practical challenges may be partially independent of academic intelligence.
• Motivation & Grit: Personality traits like conscientiousness, persistence, and growth mindset significantly impact achievement independently of cognitive ability. A highly motivated person with average IQ may outperform a less motivated person with superior IQ.
• Domain-Specific Expertise: Deep knowledge in particular fields—chess, music, medicine—involves pattern recognition and problem-solving that may not transfer across domains, even though it looks like "intelligence" within the domain.
• Wisdom: The application of intelligence and experience to make good life decisions. Wisdom involves values, judgment, and metacognition that go beyond raw cognitive ability.
• Social Intelligence: Understanding social dynamics, reading nonverbal cues, and navigating interpersonal relationships involves processes not measured by standard IQ tests.
• Physical/Kinesthetic Intelligence: Bodily coordination, athletic ability, and motor skills—while they involve cognitive components, they're not what IQ tests measure.

5 How IQ Scores Are Standardized

Modern IQ scores are deviation IQ scores, standardized to have a mean of 100 and standard deviation of 15. This provides a consistent scale for interpretation:

This standardization means IQ scores are always relative to a comparison group—typically same-age peers from the norming sample. A score of 115 means you performed better than approximately 84% of people your age, regardless of which test was used (assuming proper norming).

ACIS uses the same standardization, allowing direct comparison with other major intelligence tests like the WAIS-V and Stanford-Binet 5. Scaled scores for individual subtests have a mean of 10 and SD of 3, with scores of 7-13 considered average range.

6 Reliability and Validity

For a test to be meaningful, it must demonstrate both reliability (consistency) and validity (accuracy in measuring what it claims to measure):

• Test-Retest Reliability: If you take the same IQ test twice (with appropriate intervals), scores should be highly correlated. Modern IQ tests achieve reliabilities of .90-.95, meaning 90-95% of variance is true score variance, not measurement error.
• Internal Consistency: Items within the same subtest should correlate with each other, indicating they measure the same construct. ACIS achieves internal consistency exceeding .95.
• Criterion Validity: IQ scores should predict real-world outcomes—academic achievement, job performance, income. Research by Schmidt & Hunter (1998) confirms correlations of .50-.70 with academic success and .30-.60 with occupational success.
• Construct Validity: The test should measure what the theory says it measures. Factor analysis should reveal the expected structure (g plus broad abilities). ACIS demonstrates clear factor structure aligned with CHC theory.
• Convergent Validity: Different IQ tests purporting to measure the same thing should correlate highly. Major IQ tests correlate .80-.95 with each other.

ACIS was developed with rigorous attention to these psychometric properties, ensuring that scores are stable, meaningful, and interpretable.

7 The Standard Error of Measurement

No psychological measurement is perfectly precise. The Standard Error of Measurement (SEM) quantifies how much scores might vary due to random factors:

For ACIS (with reliability >.95), the SEM for Full Scale IQ is approximately 3-4 points. This means a true score of 115 might produce observed scores anywhere from roughly 111-119 across repeated testings.

This has important implications:

• Small score differences (5 points or less) may not be meaningful
• Confidence intervals are more informative than point estimates
• A score of 128 vs. 132 doesn't reliably distinguish ability levels
• Score ranges should be interpreted, not exact numbers

When interpreting your ACIS results, consider scores within 5-7 points of each other as essentially equivalent, and focus on larger patterns in your cognitive profile.

8 The Value of a Cognitive Profile

Beyond a single IQ number, ACIS provides a detailed cognitive profile showing relative strengths and weaknesses across domains. This profile reveals information a composite score cannot:

• Personal Strengths: Areas where you perform above your own average—these represent talents to leverage
• Development Areas: Domains where you're weaker relative to yourself—may benefit from targeted strategies or accommodations
• Learning Style Indicators: Strong verbal with weaker spatial suggests preference for text-based learning; the reverse suggests preference for diagrams and visual materials
• Career Guidance: Certain profiles align better with different occupations—high spatial suggests engineering/architecture; high verbal suggests law/writing/teaching
• Clinical Indicators: Large discrepancies between domains can sometimes suggest learning disabilities, processing differences, or conditions worth investigating
• Educational Planning: Knowing your child's cognitive profile helps select appropriate schools, enrichment activities, and support services

ACIS provides index scores for six cognitive domains plus an overall FSIQ, and scaled scores for all 20 subtests. This granular information enables much more nuanced interpretation than simple "your IQ is X" feedback.

9 Common Misconceptions

Several persistent myths about IQ testing deserve clarification:

• "IQ is fixed from birth": While IQ has substantial heritability (~80% in adulthood, per Plomin & Deary, 2015), it's not completely fixed. Early interventions, education quality, and cognitive engagement can influence development, especially in childhood.
• "IQ tests are culturally biased": Modern IQ tests are carefully constructed to minimize bias. Statistical techniques (differential item functioning analysis) identify and remove biased items. Tests like ACIS use many non-verbal tasks that don't require specific cultural knowledge. However, extreme cultural/linguistic differences from the norming sample may affect validity.
• "High IQ guarantees success": IQ is a strong predictor of many outcomes but not the only factor. Personality, opportunity, motivation, social skills, and luck all matter.
• "IQ tests only measure test-taking ability": While test-taking skills can influence scores modestly, they cannot create high scores without genuine cognitive ability. The predictive validity of IQ for real-world outcomes demonstrates it measures something meaningful beyond test performance.
• "Online IQ tests are meaningless": While many free online tests are poorly constructed, professionally developed assessments like ACIS can provide valid measurements when properly designed and normed.

10 Summary: What Your ACIS Score Means

Your ACIS results tell you several things:

• Full Scale IQ: Your overall cognitive ability relative to same-age peers—a summary statistic of general intellectual capacity
• Index Scores: Your abilities in six major cognitive domains—revealing your unique pattern of strengths and weaknesses
• Scaled Scores: Performance on individual subtests—the most granular level of analysis
• Percentile Rankings: Where you stand relative to the population—useful for understanding rarity of abilities
• Confidence Intervals: The range in which your true score likely falls—accounting for measurement error

Use this information for self-understanding, educational planning, career exploration, and identifying areas where you might want to develop compensatory strategies. Remember that IQ is one dimension of who you are—important, but not the whole picture.

11 Key Research References

For readers who want primary sources and authoritative overviews:

• Spearman, C. (1904). General intelligence, objectively determined and measured. American Journal of Psychology.
• Carroll, J.B. (1993). Human Cognitive Abilities: A Survey of Factor-Analytic Studies. Cambridge University Press.
• Horn, J.L. & Cattell, R.B. (1966). Refinement and test of the theory of fluid and crystallized general intelligences. Journal of Educational Psychology, 57(5), 253-270.
• McGrew, K.S. (2009). CHC theory and the human cognitive abilities project. Intelligence, 37(1), 1-10.
• Flanagan, D.P. & Harrison, P.L. (2012). Contemporary Intellectual Assessment (3rd ed.). Guilford Press.
• Deary, I.J., Penke, L. & Johnson, W. (2010). The neuroscience of human intelligence differences. Nature Reviews Neuroscience, 11, 201-211.
• Neisser, U. et al. (1996). Intelligence: Knowns and unknowns. American Psychologist, 51(2), 77-101.
• Nisbett, R.E. et al. (2012). Intelligence: New findings and theoretical developments. American Psychologist, 67(2), 130-159.
• Plomin, R. & Deary, I.J. (2015). Genetics and intelligence differences: Five special findings. Nature Reviews Genetics, 16, 359-371.
• Conway, A.R.A. et al. (2003). Working memory capacity and its relation to general intelligence. Psychological Bulletin.
• Baddeley, A. (2003). Working memory: Looking back and looking forward. Nature Reviews Neuroscience, 4, 829-839.
• AERA, APA, & NCME (2014). Standards for Educational and Psychological Testing.