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IQ Test Scores: The Basics of IQ Score Interpretation

“What do you mean my child isn’t gifted — he got 99 on those tests! That’s nearly a perfect score, isn’t it?”

“The criteria you handed out says ‘a score in the 97th percentile or above.’ Peter got an IQ score of 97! That meets the requirement, doesn’t it?”

Comments like these are not unusual and indicate a complete misunderstanding of IQ test 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:


Descriptive Classifications of Intelligence Quotients
 
IQ Description% of Population
130+Very superior2.2%
120-129Superior6.7%
110-119High average16.1%
90-109Average50%
80-89Low average16.1%
70-79Borderline6.7%
Below 70Extremely low2.2%

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:

140Top Civil Servants; Professors and Research Scientists.
130Physicians and Surgeons; Lawyers; Engineers (Civil and Mechanical)
120School Teachers; Pharmacists; Accountants; Nurses; Stenographers; Managers.
110Foremen; Clerks; Telephone Operators; Salesmen; Policemen; Electricians.
100+Machine Operators; Shopkeepers; Butchers; Welders; Sheet Metal Workers.
100-Warehousemen; Carpenters; Cooks and Bakers; Small Farmers; Truck and Van Drivers.
90Laborers; Gardeners; Upholsterers; Farmhands; Miners; Factory Packers and Sorters.
 

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 test 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:

IQPercentile
6501
7002
7505
8009
8516
9025
9537
10050
10563
11075
11584
12091
12595
13098
13599

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.


Be Cautious!

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.

References:

  • Engle, T. L., & Snellgrove, L., Psychology: Its Principles and Applications (6th ed.), (New York: Harcourt Brace Jovanovich, Inc., 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 (London: Futura Publications, 1977).