How Are Scores on IQ Tests Calculated?

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You've probably heard of IQ tests and understand that they measure intelligence. People talk a lot about IQ scores, but the fact is that many people are not quite sure what these scores really mean. What exactly is a "high" IQ score? What is an average IQ? What kind of score does it take to be considered a genius?

In order to understand what all these numbers really mean, it is important to understand how scores on these tests are calculated. While many tests utilize similar methods to derive their scores, it is also important to note that each test is different and scoring methods may not be the same from one test to another.

IQ Tests Are Standardized

In order to adequately assess and interpret test scores, scientists who develop IQ tests use a process known as standardization. What exactly does this mean?

The standardization process involves administering the test to a representative sample of the entire population that will eventually take the test. This initial sample represents the total population as accurately as possible and reflects many of the things that are present in the general population.

For example, a representative sample should include the same percentage of individuals of certain sexes and ages as one would find in the total population. 

Each test taker completes the test under the same conditions as all other participants in the sample group. This process allows IQ test developers to establish norms, or standards, by which individual scores can be compared.

Most IQ Tests Are Normally Distributed

Intelligence test scores typically follow what is known as a normal distribution, a bell-shaped curve in which the majority of scores lie near or around the average score. For example, the majority of scores (about 68%) on the Wechsler series of intelligence tests tend to lie between plus 15 or minus 15 points from the average score of 100.

This means that approximately 68% of people who take this test will score somewhere between 85 and 115. As you look further toward the extreme ends of the distribution, scores tend to become less common.

How common are very high IQ scores?

Very few individuals (approximately 0.2%) receive a score of more than 145 (indicating a very high IQ) or less than 55 (indicating a very low IQ) on the test.

In many cases, an IQ score that falls below 70 is considered low IQ, while a score of about 140 indicates high IQ. In the past, scores below 70 were used as a marker to identify intellectual disabilities. Today, test scores alone are not enough to diagnose an intellectual disability and diagnosticians also consider factors such as the age of onset and adaptive skills.

A Closer Look at IQ Scores

The following is a rough breakdown of various IQ score ranges. Some tests present scores differently and with differing interpretations of what those scores might mean.

However, it is important to remember that IQ tests are only one measure of intelligence. Many experts suggest that other important elements contribute to intelligence, including social and emotional factors. Some experts even suggest that these social and emotional skills actually matter more than IQ when it comes to determining success in life. 

IQ Classifications
IQ Level Descriptive Classification
130+ Very Superior
120 to 129 Superior
110 to 119 High Average
90 to 109 Average
80 to 89 Low Average
70 to 79 Borderline
69 & below Intellectual Disability
These classifications come from the Wechsler series of IQ tests for children and adults.

What Do IQ Tests Measure?

There are a number of different intelligence tests in existence and their content can vary considerably. French psychologist Alfred Binet was the first to develop a formal test of intelligence and a form of his original test is still in use today as the Stanford-Binet intelligence test. Later, psychologist Charles Spearman developed a concept of general intelligence, or a general mental ability to perform a wide variety of cognitive tasks.

Modern intelligence tests often focus on abilities such as mathematical skills, memory, spatial perception, and language abilities. The capacity to see relationships, solve problems, and remember information are important components of intelligence, so these are often the skills on which IQ tests focus.

Some commonly used intelligence tests include the Wechsler Adult Intelligence Scale, the Wechsler Intelligence Scale for Children, the Stanford-Binet, the Cognitive Assessment System, the Kaufman Assessment Battery for Children, and the Woodcock-Johnson Tests of Cognitive Abilities.

Important Points to Remember

A few key things to remember about how IQ scores are calculated:

  • IQ test scores are normally distributed and follow a roughly bell-shaped curve
  • Modern IQ tests compare individuals of the same age group
  • Reliable IQ tests should provide consistent results. People should score roughly the same each time they take the test.
  • While higher IQ scores are linked to increased health, academic performance, and overall well-being, these scores do not necessarily predict an individual's success in life.

A Word From Verywell

IQ tests are among the most commonly administered psychological tests. In order to understand what these scores really mean, it is essential to look at exactly how these test scores are calculated. Today, many tests are standardized and scores are derived by comparing individual performance against the norms for the individual's age group.

While IQ scores can reveal information about an individual's abilities in certain domains, it is also important to remember that other factors, including such things as adaptive skills, emotional intelligence, and task performance, are also important indicators of an individual's capabilities.

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Article Sources
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  1. Kaufman A. IQ Testing 101. New York: Springer; 2010.

  2. Bartholomew D. Measuring Intelligence. Cambridge, UK: Cambridge University Press; 2004.

  3. Stangor C. 10.1 Defining and Measuring Intelligence. Introduction to Psychology. Version 2.1. Published July 2014.

Additional Reading
  • Breedlove, S.M. Principles of Psychology. Sunderland, MA: Sinauer Associates, Inc; 2015.