IQ tests attempt to measure and provide an intelligence quotient, which is a score derived from a standardized test designed to access human intelligence. There are now several variations of these tests that have built upon and expanded the original test, which was designed to identify children in need of remedial education. Currently, IQ tests are used to study distributions in scores among specific populations. Over time, these scores have come to be associated with differences in other variables such as behavior, performance, and well-being; these vary based on cultural norms.

Measuring IQ Scores

After decades of revision, modern IQ tests produce a mathematical score based on standard deviation, or difference from the average score. Scores on IQ tests tend to form a bell curve with a normal distribution. In a normal distribution, 50% of the scores will be below the average (or mean) score and 50% of the scores will be above it. 

IQ Bell Curve

In a normally distributed bell curve, half the scores are above the mean and half are below. The farther from the mean, the less frequent a given score is.

Normal distributions are special, because their data follows a specific, reliable pattern. Standard deviation is a term for measuring how far a given score is from the mean; in any normal distribution, you can tell what percentage of a population will fall within a certain score range by looking at standard deviations. It is a statistical law that under a normal curve, 68% of scores will lie between -1 and +1 standard deviation, 95% of scores will lie between -2 and +2 standard deviations, and >99% percent of scores will fall between -3 and +3 standard deviations.

The scores of an IQ test are normally distributed so that one standard deviation is equal to 15 points; that is to say, when you go one standard deviation above the mean of 100, you get a score of 115. When you go one standard deviation below the mean, you get a score of 85. Two standard deviations are 30 points above or below the mean, three are 45 points, and so on. So by current measurement standards, 68% of people score between 85 and 115, 95% of the population score between 70 and 130 points, and over 99% of the population score between 55 and 145.

It should be noted that this standard of measure does not imply a linear relationship between IQ and mental ability: a person with a score of 50 does not have half the mental ability of a person with a score of 100.

Standard Deviations of IQ Scores

IQ test scores tend to form a bell curve, with approximately 95% of the population scoring between two standard deviations of the mean score of 100.

IQ tests are a type of psychometric (person-centric) testing thought to have very high statistical reliability. This means that while a person's scores may vary slightly with age and environmental condition, they are repeatable and will generally agree with one another over time. They are also thought to have high statistical validity, which means that they measure what they actually claim to measure, intelligence. This means that many people trust them to be used in other applications, such as clinical or educational purposes.

Types of IQ Tests and Tasks

There are a wide variety of IQ tests that use slightly different tasks and measures to calculate an overall IQ score. The most commonly used test series is the Wechsler Adult Intelligence Scale (WAIS) and its counterpart, the Wechsler Intelligence Scale for Children (WISC). Other commonly used tests include the original and updated version of Stanford-Binet, the Woodcock-Johnson Tests of Cognitive Abilities, the Kaufman Assessment Battery for Children, the Cognitive Assessment System, and the Differential Ability Scale. While all of these tests measure intelligence, not all of them label their standard scores as IQ scores.

WAIS Test Components

The WAIS uses a variety of components to determine a person's IQ score, including verbal, memory, perceptual, and processing skills.

Currently, most tests tend to measure both verbal and performance IQ. Verbal IQ is measured through both comprehension and working (short-term) memory skills, such as vocabulary and arithmetic. Performance IQ is measured through perception and processing skills, such as matrix completion and symbol coding. All of these measures and tasks are used to calculate a person's IQ.

Sample IQ Test Question

This is a sample IQ test question modeled after a person's ability to identify and continue patterns in progressive matrices.