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An intelligence quotient, or IQ, is a numerical value assigned to an individual based on his or her performance on one or more standardized tests which are designed to determine or assess the person's inherent or genetic intelligence. The range for an IQ starts at zero and, in theory, has no upper limit, although the highest IQ scores ever recorded are around 200. Statistics show that approximately 95% of the population have IQ scores between 70 and 130, making a "normal IQ" 100.
Although there are an infinite number of tests that claim to test a person's IQ, there are a few that are widely used and recognized. The Stanford-Binet and the Wechsler Adult Intelligence Scale and Intelligence Scale for Children are just a few reputable tests used to determine whether someone has a normal IQ. Studies indicate that, although test scores may vary somewhat from one test to the other, scores tend to stay relatively constant regardless of the test taken.
In terms of what is considered a normal IQ, scientists have determined that 95% of all test takers score within two standard deviations of the mean. For purposes of an IQ test, one standard deviation is 15 points. The median score is set at 100, which means that the vast majority of all test takers score within 30 points on either side of the median — or between 70 and 130. Although 100 is the median, most experts consider a "normal IQ" to be anything within one to two deviations of the median, or in the 70 to 130 range.
When a test taker's IQ surpasses the normal IQ range, he or she is usually considered to be "gifted" or to have a "genius" IQ. Opinions differ with regard to where "normal" leaves off and "gifted" and "genius" begin. The general consensus, however, puts tests takers who score above 115 to 120 in the "gifted" category and those who score above 130 to 140 in the "genius" category.
Likewise, test takers who score below 70 are also considered to be outside the normal IQ range. An IQ score below 70 is considered to be a sign of mental retardation, or intellectual disability, by most experts. Many experts consider a score between 70 and 80 to be at the lower range, or borderline range, of intellectual functioning. Scores below 70 indicate mild to severe mental retardation, or the inability to function at a normal intellectual level.