How Are Scores on IQ Tests Calculated?

Normal distribution of IQ scores
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How exactly are IQ test scores calculated? We 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 adequately assess and interpret test scores, psychometricians use a process known as standardization. The standardization process involves administering the test to a representative sample of the entire population that will eventually take the test.

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

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 percent) on the WAIS-III tend to lie between plus 15 or minus 15 points from the average score of 100.

This means that approximately 68 percent 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.

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 about 140 indicates high IQ. In the past, scores below 70 were used as a marker to identify intellectual disabilities and mental retardation. Today, test scores alone are not enough to diagnose an intellectual disability and diagnosticians also consider factors such as 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.

  • 1 to 70 - Mental disability
  • 71 to 84 - Borderline mental disability
  • 85 to 114 - Average intelligence
  • 115 to 129 - Above average; bright
  • 130 to 144 - Moderately gifted
  • 145 to 159 - Highly gifted
  • 160 to 179 - Exceptionally gifted
  • 180 and up - Profoundly gifted

What Do IQ Test 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 skills. The ability to see relationships, solve problems and remember information are important components of intelligence, so these are often the skills on which IQ tests are focused.

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
  • Modern IQ tests compare individuals of the same age group
  • Reliable IQ tests should provide consistent results
  • While higher IQ scores are linked to increased health, academic performance and overall well-being, these scores do not necessarily predict and individuals success in life.

References

Barthalomew, D. J. (2004). Measuring Intelligence: Facts and Fallacies. Cambridge: Cambridge University Press.

Breedlove, S.M. (2015). Principles of Psychology. Sunderland, MA: Sinauer Associates, Inc.

Kaufman, A.S. (2009). IQ Testing 101. New York: Springer Publishing.

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