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IQ testing is a method used by psychologists to measure what is generally considered intelligence. The concept of IQ, or "Intelligence Quotient" was first introduced by French psychologist Alfred Binet in 1904. The "quotient" refers to Binet's definition of IQ as (Mental Age) divided by (Chronological Age) or M.A./C.A. This quotient is then multiplied by 100 to make it a whole number. An 8 year old child with the mental ability of a 12 year old has a mental age which is 1.50 times his chronological age (12/8 = 1.5). Multiplying this quotient by 100 gives the child's ratio IQ: 150. Using this method, a child functioning at the average level for her age would obtain an IQ of 100.
Most of the abilities measured by an IQ test tend to level off around age 16, so this method does not work for adults. To convert a mentally retarded adult's IQ into a rough age equivalent, multiply the IQ by 16, and then divide by 100. So an adult with a 50 IQ is functioning at roughly an 8-year-old level.
Modern IQ tests use a "deviation IQ" rather than a ratio IQ. With this method, test takers are referenced to other people of their own age. The average IQ is still 100, but deviations from the average are assigned a number which corresponds to a percentile rank.
Most IQ tests consist of subtests measuring various qualities, such as factual knowledge, short-term memory, abstract reasoning, visual-spatial abilities, and common sense. Intelligence is always measured relative to a particular culture; "culture free" tests of intelligence do not exist. IQ tests do a good job of predicting academic success. They are not good at measuring such qualities as interpersonal skill or creativity. Although IQ scores tend to be fairly stable, IQ will vary over time. The Wechsler tests are the most common individually administered IQ tests. They currently include the WISC-IV (age 6-16 years), the WAIS-IV (age 16-89 years), and the WPPSI-III (age 2.5 - 7 years). Shown below are the labels and frequency of Wechsler IQ scores. Keep in mind that, due to random factors, IQ scores can vary about 5 points from week to week, and can often change by 10 points or even more over a period of years.
Information: Similar to "Trivial Pursuit," this subtest measures fund of factual information. It is strongly influenced by culture. An American education and intact long-term memory will contribute to a higher score. Sample question (not really on the tests): "What is the capital of France?"
Comprehension: This subtest measures understanding of social conventions and common sense. It is also culturally loaded. Sample question: "What is the thing to do if you find an injured person laying on the sidewalk?"
Digit Span: Requires the repetition of number strings forward and backwards. Measures concentration, attention, and immediate memory. Lower scores are obtained by persons with an attention deficit or anxiety.
Similarities: This subtest measures verbal abstract reasoning and conceptualization abilities. The individual is asked how two things are alike. Sample question: "How are a snake and an alligator alike?"
Vocabulary: This test measures receptive and expressive vocabulary. It is the best overall measure of general intelligence (assuming the test-taker's native language is English). Sample question: "What is the meaning of the word 'articulate'?"
Arithmetic: Consists of mathematical word problems which are performed mentally. Measures attention, concentration, and numeric reasoning. Sample question: "John bought three books for five dollars each, and paid ten percent sales tax. How much did he pay all together?"
Object Assembly: Consists of jigsaw puzzles. Measures visual-spatial abilities and ability to see how parts make up a whole (this subtest is optional on the revised Weschler tests).
Block Design: One of the strongest measures of nonverbal intelligence and reasoning. Consists of colored blocks which are put together to make designs.
Digit Symbol/Coding/Animal House: Symbols are matched with numbers or shapes according to a key. Measures visual-motor speed and short-term visual memory.
Picture Arrangement: Requires that pictures be arranged in order to tell a story. Measures nonverbal understanding of social interaction and ability to reason sequentially.
Picture Concepts: A new subtest on the WISC-IV. Requires matching pictures which belong together based on common characteristics. Measures non-verbal concept formation and reasoning; a non-verbal counterpart of Similarities.
Picture Completion: Requires recognition of the missing part in pictures. Measures visual perception, long-term visual memory, and the ability to differentiate essential from inessential details.
Matrix Reasoning: (WAIS-III only) Modeled after Raven's Progressive Matrices, this is an untimed test which measures abstract nonverbal reasoning ability. It consists of a sequence or group of designs, and the individual is required to fill in a missing design from a number of choices.
The WAIS-IV was released in January 2009. Thanks to Ann Simun, PsyD for the following information on the new edition:
Omitted: Object Assembly; Picture Arrangement; Memory Tasks for Coding
Added: Figure Weights: A visual measure of fluid intelligence, with complex verbal instructions; a pointing response is allowed. The subject uses logic to determine equivalence of figures, using a drawing of a scale; problems are presented sequentially. Untimed. Measures quantitative and analytical reasoning. Loads on Perceptual Organization Index.
Visual Puzzles: Measures visual spatial reasoning, whole part integration, and mental rotation. A figure is presented, and underneath are a 6 choices. The subject is asked, "Which 3 of these pieces go together to make this puzzle?" Mental flexibility and rule following is also involved, as some choices involve a 2 piece solution, which would be incorrect. Verbal directions, nonverbal responding allowed (pointing response). Loads on Perceptual Organization Index.
Cancellation: A speeded visual scanning task, with a pencil mark response; visual discrimination, color discrimination, information processing speed, and pencil use speed are required. Loads on Processing Speed Index.
There are several other common individually-administered psychological tests. The Stanford-Binet was the first test to use the concept of IQ. It is now in its 5th edition and reports intelligence as four scores: Verbal Reasoning, Abstract/Visual, Quantitative, and Short-term Memory. The Stanford-Binet covers the range from age 2.5 to young adult. The Leiter test (now revised) covers the same age range. It has the benefit of being completely non-verbal, and thus can be used to test an individual who does not speak English. Other non-verbal tests include Raven's Progressive Matrices and the Test of Nonverbal Intelligence (TONI-III).