An intelligence quotient, or IQ, test does not directly measure intelligence, but rather components of intelligence such as spatial reasoning, vocabulary and visualization. Different tests emphasize different areas of intelligence. IQ test scores are defined in relationship to an average of 100. However, different versions of the IQ test place different emphases on scores farther from 100.
Calculation in Relation to 100
The standard of scoring on an IQ test is based on a scale from zero to 200 that is based on the general average score in the larger population. IQ scores are defined so the average score for a population is 100; the most common scores will also fall near or around 100. A graph of the most common IQ scores will appear as a hill with its peak at 100. This is called a bell curve. So for any IQ scale, normal intelligence will be around 100. If you score 100 on an IQ test, you're in the 50th percentile of scoring. This means you've scored higher than 50 percent of people who've taken the test.
The Stanford-Binet test and scale was developed in the early 20th century to evaluate children's intelligence. A Stanford-Binet test measures five qualities of intelligence: knowledge, fluid reasoning, quantitative reasoning, visual-spatial processing and working memory. In the Stanford-Binet scale, levels of intelligence are defined by their distance from 100. Average intelligence is defined between 90 and 110 points, with 80 to 89 being defined as "dullness" and 110 to 120 as "superior intelligence." A genius level score on the Stanford-Binet scale is over 140, while scores under 70 indicate mental retardation.
The Wechsler Adult Intelligence Scale was created in the 1950s to measure intelligence in adults and adolescents. Both the Wechsler and Stanford-Binet scales use a standard deviation of 15 points from the average. This means that 68 percent of scores fall between 85 and 115, and 95 percent of scores fall between 70 and 130. Only 1 percent of scores fall below 15 or above 145. Wechsler's scale measures four areas of intelligence: verbal comprehension, perceptual reasoning, working memory and processing speed, which are combined to provide a total IQ score. Scores between 80 and 119 are defined as average, with scores in the 80s as "low average" and scores in the 110s as "high average." The Wechsler scale does not include a "genius" level, but defines scores over 130 as "very superior."
The Cattell Culture Fair III Test is designed to test intelligence while avoiding a bias toward Western students. The Cattell test, along with the Wechsler, is used as qualification for Mensa. The Cattell Scale, like other IQ scales, places its average score at 100. However, it uses a standard deviation of 24, rather than 15, in its scoring system, allowing it to provide a more accurate assessment of scores that are farther from 100. The Cattell scale defines average intelligence between 100 and 119, with scores in the 90s as "below average" and those in the 120s and 130s as "above average." Genius level IQ falls above 160 points.