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IQ Scores: IQ Score Interpretation

IQ scores are often misunderstood. Learn the basics of IQ score interpretation in this article.

Intelligence testing began in earnest in France, when in 1904 psychologist Alfred Binet was commissioned by the French government to find a method to differentiate between children who were intellectually normal and those who were inferior. The purpose was to put the latter into special schools. There they would receive more individual attention and the disruption they caused in the education of intellectually normal children could be avoided. 

This led to the development of the Binet Scale, also known as the Simon-Binet Scale in recognition of Theophile Simon's assistance in its development. The test had children do tasks such as follow commands, copy patterns, name objects, and put things in order or arrange them properly. Binet gave the test to Paris schoolchildren and created a standard based on his data. Following Binet’s work, the phrase “intelligence quotient,” or “IQ,” entered the vocabulary.  

Lewis M. Terman worked on revising the Simon-Binet Scale. His final product, published in 1916 as the Stanford Revision of the Binet-Simon Scale of Intelligence (also known as the Stanford-Binet), became the standard intelligence test in the United States for the next several decades. By the 1920s mass use of the Stanford-Binet Scale and other tests had created a multimillion-dollar testing industry. 

Despite the fact that the IQ test industry is already a century old, IQ scores are still often misunderstood. Comments like, “What do you mean my child isn’t gifted — he got 99 on those tests! That’s nearly a perfect score, isn’t it?” or “The criteria you handed out says ‘a score in the 97th percentile or above.’ Susan got an IQ score of 97! That meets the requirement, doesn’t it?” are not unusual and indicate a complete misunderstanding of IQ scores.

Understanding IQ Scores

IQ stands for intelligence quotient. Supposedly, it is a score that tells one how “bright” a person is compared to other people. The average IQ is by definition 100; scores above 100 indicate a higher than average IQ and scores below 100 indicate a lower that average IQ. Theoretically, scores can range any amount below or above 100, but in practice they do not meaningfully go much below 50 or above 150. 

Half of the population have IQ’s of between 90 and 110, while 25% have higher IQ’s and 25% have lower IQ’s: 

Apparently, the IQ gives a good indication of the occupational group that a person will end up in, though not of course the specific occupation. In their book, Know Your Child’s IQ, Glen Wilson and Diana Grylls outline occupations typical of various IQ levels: 

IQ Expressed in Percentiles

IQ is often expressed in percentiles, which is not the same as percentage scores, and a common reason for the misunderstanding of IQ scores. Percentage refers to the number of items which a child answers correctly compared to the total number of items presented. If a child answers 25 questions correctly on a 50 question test he would earn a percentage score of 50. If he answers 40 questions on the same test his percentage score would be 80. Percentile, however, refers to the number of other test takers’ scores that an individual’s score equals or exceeds. If a child answered 25 questions and did better than 50% of the children taking the test he would score at the 50th percentile. However, if he answered 40 questions on the 50 item test and everyone else answered more than he did, he would fall at a very low percentile — even though he answered 80% of the questions correctly. 

On most standardized tests, an IQ of 100 is at the 50th percentile. Most of our IQ tests are standardized with a mean score of 100 and a standard deviation of 15. What that means is that the following IQ scores will be roughly equivalent to the following percentiles: 

An IQ of 120 therefore implies that the testee is brighter than about 91% of the population, while 130 puts a person ahead of 98% of people. A person with an IQ of 80 is brighter than only 9% of people, and only a few score less than 60.

It is necessary to be very cautious in using a descriptive classification of IQ’s. The IQ is, at best, a rough measure of academic intelligence. It certainly would be unscientific to say that an individual with an IQ of 110 is of high average intelligence, while an individual with an IQ of 109 is of only average intelligence. Such a strict classification of intellectual abilities would fail to take account of social elements such as home, school, and community. These elements are not adequately measured by present intelligence tests. Furthermore, it would not take account of the fact that an individual may vary in his test score from one test to another. 

Measures of intelligence may be valuable — although the value is often overrated — but much harm can be done by persons who try to classify individuals strictly on the basis of such measures alone. No one should be either alarmed or discouraged if he finds that his IQ is not as high as he might have hoped. Remember that many elements besides IQ contribute to success and happiness. Also note that IQ is not a fixed quantity, but can be increased by means of education. This was demonstrated by the Milwaukee project as well as numerous other research studies.

The Milwaukee Project

In the late 1960s, under the supervision of Rick Heber of the University of Wisconsin, a project was begun to study the effects of intellectual stimulation on children from deprived environments. In order to find a “deprived environment” from which to draw appropriate subjects for the study, Heber and his colleagues examined the statistics of different districts within the city of Milwaukee. One district in particular stood out. The residents of this district had the lowest median income and lowest level of education to be found in the city. This district also had the highest population density and rate of unemployment of any area of Milwaukee. There was one more statistic that really attracted Heber’s attention: Although this district contained only 3 percent of the city’s population, it accounted for 33 percent of the children in Milwaukee who had been labeled “mentally retarded”! 

At the beginning of the project, Heber selected forty newborns from the depressed area of Milwaukee he had chosen. The mothers of the infants selected all had IQ’s below 80. As it turned out, all of the children in the study were black, and in many cases the fathers were absent. The forty newborns were randomly assigned, 20 to an experimental group and 20 to a control group. 

Both the experimental group and the control group were tested an equal number of times throughout the project. An independent testing service was used in order to eliminate possible biases on the part of the project members. In terms of physical or medical variables, there were no observable differences between the two groups. 

The experimental group entered a special program. Mothers of the experimental group children received education, vocational rehabilitation, and training in homemaking and child care. The children themselves received personalized enrichment in their home environments for the first three months of their lives, and then their training continued at a special center, five days a week, seven hours a day, until they were ready to begin first grade. The program at the center focused upon developing the language and cognitive skills of the experimental group children. The control group did not receive special education or home-based intervention and enrichment. 

By the age of six all the children in the experimental group were dramatically superior to the children in the control group. This was true on all test measures, especially those dealing with language skills or problem solving. The experimental group had an IQ average of 120.7 as compared with the control group’s 87.2! 

At the age of six the children left the center to attend the local school. By the time both groups were ten years old and in fifth grade, the IQ scores of the children in the experimental group had decreased to an average of 105 while the control group’s average score held steady at about 85. One possible reason for the decline is that schooling was geared for the slower students. The brighter children were not given materials suitable for their abilities and they began to fall back. Also, while the experimental children were in the special project center for the first six years they ate well, receiving three hot, balanced meals a day. Once they left the center and began to attend the local school, many reported going to classes hungry, without breakfast or a hot lunch.

Sources:

• Dworetzky, J. P., Introduction to Child Development (St. Paul: West Publishing Company, 1981).
• Engle, T. L., & Snellgrove, L., Psychology: Its Principles and Applications (6th ed.), (Harcourt Brace Jovanovich, Inc.: New York, 1974).
• Swiegers, D. J., & Louw, D. A., “Intelligensie,” in D. A. Louw (ed.), Inleiding tot die Psigologie (2nd ed.), (Johannesburg: McGraw Hill, 1982).
• “Test Score Interpretation,” Hampton City Schools, Psychological Services.
• Wilson, G., & Grylls, D., Know Your Child’s IQ (Futura Publications: London, 1977).