IQ classification

IQ classification is the practice of categorizing human intelligence, as measured by intelligence quotient tests, into categories such as "superior" and "average".
With the usual IQ scoring methods, an IQ score of 100 means that the test-taker's performance on the test is of average performance in the sample of test-takers of about the same age as was used to norm the test. An IQ score of 115 means performance one standard deviation above the mean, while a score of 85 means performance one standard deviation below the mean, and so on. This "deviation IQ" method is used for standard scoring of all IQ tests in large part because they allow a consistent definition of IQ for both children and adults. By the existing "deviation IQ" definition of IQ test standard scores, about two-thirds of all test-takers obtain scores from 85 to 115, and about 5 percent of the population scores above 125.
When IQ testing was first created, Lewis Terman and other early developers of IQ tests noticed that most child IQ scores come out to approximately the same number regardless of testing procedure. Variability in scores can occur when the same individual takes the same test more than once. Further, a minor divergence in scores can be observed when an individual takes tests provided by different publishers at the same age. There is no standard naming or definition scheme employed universally by all test publishers for IQ score classifications.
Even before IQ tests were invented, there were attempts to classify people into intelligence categories by observing their behavior in daily life. Those other forms of behavioral observation were historically important for validating classifications based primarily on IQ test scores. Some early intelligence classifications by IQ testing depended on the definition of "intelligence" used in a particular case. Contemporary IQ test publishers take into account reliability and error of estimation in the classification procedure.

Differences in individual IQ classification

Pupil	KABC-II	WISC-III	WJ-III
Asher	90	95	111
Brianna	125	110	105
Colin	100	93	101
Danica	116	127	118
Elpha	93	105	93
Fritz	106	105	105
Georgi	95	100	90
Hector	112	113	103
Imelda	104	96	97
Jose	101	99	86
Keoku	81	78	75
Leo	116	124	102

IQ tests generally are reliable enough that most people 10 years of age and older have similar IQ scores throughout life. Still, some individuals score very differently when taking the same test at different times or when taking more than one kind of IQ test at the same age. About 42% of children change their score by 5 or more points when re-tested.
For example, many children in the famous longitudinal Genetic Studies of Genius begun in 1921 by Lewis Terman showed declines in IQ as they grew up.
Terman recruited school pupils based on referrals from teachers, and gave them his Stanford–Binet IQ test. Children with an IQ above 140 by that test were included in the study. There were 643 children in the main study group. When the students who could be contacted again were retested at high school age, they were found to have dropped 9 IQ points on average in Stanford–Binet IQ. Some children dropped by 15 IQ points or by 25 points or more. Yet parents of those children thought that the children were still as bright as ever, or even brighter.
Because all IQ tests have error of measurement in the test-taker's IQ score, a test-giver should always inform the test-taker of the confidence interval around the score obtained on a given occasion of taking each test. IQ scores are ordinal scores and are not expressed in an interval measurement unit. Besides the reported error interval around IQ test scores, an IQ score could be misleading if a test-giver failed to follow standardized administration and scoring procedures. In cases of test-giver mistakes, the usual result is that tests are scored too leniently, giving the test-taker a higher IQ score than the test-taker's performance justifies. On the other hand, some test-givers err by showing a "halo effect", with low-IQ individuals receiving IQ scores even lower than if standardized procedures were followed, while high-IQ individuals receive inflated IQ scores.
The categories of IQ vary between IQ test publishers as the category labels for IQ score ranges are specific to each brand of test. The test publishers do not have a uniform practice of labeling IQ score ranges, nor do they have a consistent practice of dividing up IQ score ranges into categories of the same size or with the same boundary scores. Thus psychologists should specify which test was given when reporting a test-taker's IQ category if not reporting the raw IQ score. Psychologists and IQ test authors recommend that psychologists adopt the terminology of each test publisher when reporting IQ score ranges.
IQ classifications from IQ testing are not the last word on how a test-taker will do in life, nor are they the only information to be considered for placement in school or job-training programs. There is still a dearth of information about how behavior differs between people with differing IQ scores. For placement in school programs, for medical diagnosis, and for career advising, factors other than IQ can be part of an individual assessment as well.

IQ classification tables

There are a variety of individually administered IQ tests in use. Not all report test results as "IQ", but most report a standard score with a mean score level of 100. When a test-taker scores higher or lower than the median score, the score is indicated as 15 standard score points higher or lower for each standard deviation difference higher or lower in the test-taker's performance on the test item content.

Wechsler Intelligence Scales

The Wechsler intelligence scales were developed from earlier intelligence scales by David Wechsler. David Wechsler, using the clinical and statistical skills he gained under Charles Spearman and as a World War I psychology examiner, crafted a series of intelligence tests. These eventually surpassed other such measures, becoming the most widely used and popular intelligence assessment tools for many years. The first Wechsler test published was the Wechsler–Bellevue Scale in 1939. The Wechsler IQ tests for children and for adults are the most frequently used individual IQ tests in the English-speaking world and in their translated versions are perhaps the most widely used IQ tests worldwide. The Wechsler tests have long been regarded as the "gold standard" in IQ testing. The Wechsler Adult Intelligence Scale—Fourth Edition was published in 2008 by The Psychological Corporation. The Wechsler Intelligence Scale for Children—Fifth Edition was published in 2014 by The Psychological Corporation, and the Wechsler Preschool and Primary Scale of Intelligence—Fourth Edition was published in 2012 by The Psychological Corporation. Like all contemporary IQ tests, the Wechsler tests report a "deviation IQ" as the standard score for the full-scale IQ, with the norming sample mean raw score defined as IQ 100 and a score one standard deviation higher defined as IQ 115.
During the First World War in 1917, adult intelligence testing gained prominence as an instrument for assessing drafted soldiers in the United States. Robert Yerkes, an American psychologist, was assigned to devise psychometric tools to allocate recruits to different levels of military service, leading to the development of the Army Alpha and Army Beta group-based tests. The collective efforts of Binet, Simon, Terman, and Yerkes laid the groundwork for modern intelligence test series.

IQ Range	IQ Classification
130 and above	Very Superior
120–129	Superior
110–119	High Average
90–109	Average
80–89	Low Average
70–79	Borderline
69 and below	Extremely Low

IQ Range	IQ Classification
130 and above	Extremely High
120–129	Very High
110–119	High Average
90–109	Average
80–89	Low Average
70–79	Very Low
69 and below	Extremely Low

Psychologists have proposed alternative language for Wechsler IQ classifications. The term "borderline", which implies being very close to being intellectually disabled, is replaced in the alternative system by a term that doesn't imply a medical diagnosis.

Corresponding IQ Range	Classifications	More value-neutral terms
130 and above	Very superior	Upper extreme
120–129	Superior	Well above average
110–119	High average	High average
90–109	Average	Average
80–89	Low average	Below average
70–79	Borderline	Well below average
69 and below	Extremely low	Lower extreme

Stanford–Binet Intelligence Scale Fifth Edition

The fifth edition of the Stanford–Binet scales was developed by Gale H. Roid and published in 2003 by Riverside Publishing. Unlike scoring on previous versions of the Stanford–Binet test, SB5 IQ scoring is deviation scoring in which each standard deviation up or down from the norming sample median score is 15 points from the median score, IQ 100, just like the standard scoring on the Wechsler tests.

IQ Range	IQ Classification
140 and above	Very gifted or highly advanced
130–139	Gifted or very advanced
120–129	Superior
110–119	High average
90–109	Average
80–89	Low average
70–79	Borderline impaired or delayed
55–69	Mildly impaired or delayed
40–54	Moderately impaired or delayed
19-39	Profound mental retardation