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Definition of IQ

Definition of IQ

An average IQ is 100 and about 95% of IQ scores fall between 70 and 130. But what does IQ mean? What is the definition of IQ? Is there an IQ range? Find out in this article.


IQ Definition and Intelligence Testing

Definition of IQ

The term intelligence quotient (IQ) originates in Germany in the 1910s. Psychologist William Stern used the term for a scoring technique for intelligence tests that he developed at University of Breslau. He defined “intelligence quotient” as a person's developmental age (obtained by intelligence testing) divided by their chronological age. The resulting quotient was multiplied by 100 to get the IQ score. The concept of determining the developmental age was later abandoned in intelligence testing but the term of IQ was established.

These days, IQ is used as an estimate of a person's general intelligence as relative to other people. As raw scores from the standardized tests are distributed normally (i.e. Gaussian distribution), it has been agreed that the median raw score of the sample population refers to IQ 100, with one standard deviation equaling to 15 points in the IQ scale.

IQ score distribution

IQ score distribution

This means that by definition, an average person has an IQ of 100 and about 95% of IQ scores fall between 70 and 130. The IQ scale is a relative measure. People with the highest IQ are, in theory, more intelligent than those with an average IQ.

For example, IQ 132 means one has been estimated to have a higher intelligence level than 98% of people but doesn’t itself tell anything on a person's absolute problem solving or logical reasoning capabilities, but just relative to the peer group. The Mensa IQ test can only be passed by those with the highest IQ.

An example of a commonly-administered standardized test is the Wechsler Adult Intelligence Scale (WAIS). It tests fluid intelligence on a performance scale and crystallized intelligence on a verbal scale. The raw scores of both sections are added up then turned into IQ score.

IQ Score Traditional Ranking System % of population
≥ 140 Genius level > 0.4
130-139 Gifted 2.3
120-129 Very Superior Intelligence 6.4
110-119 Superior Intelligence 15
90-109 Average/Normal 47
80-89 Dullness 14
70-79 Borderline deficiency 5.8
50-69 Mild Mental Retardation 2.2
35-49 Moderate mental retardation  
20-34 Severe mental retardation  
< 20 Profound mental retardation  

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