The History of the IQ Score and IQ Test

The modern IQ test score is a mathematical transformation of a raw score on an IQ test, based on the rank of that score in a normalization sample. Modern scores are sometimes referred to as "deviance IQ", while older method age-specific scores are referred to as "ratio IQ."

The two methodologies yield similar results near the middle of the bell curve, but the older ratio IQs yielded far higher scores for the intellectually gifted— for example, Marilyn vos Savant, who appeared in the Guinness Book of World Records, obtained a ratio IQ of 228. While this score could make sense using Binet's formula (and even then, only for a child), on the Gaussian curve model it would be an exceptional 7.9 standard deviations above the mean and hence virtually impossible in a population with a normal IQ distribution (see normal distribution). In addition, IQ tests like the Wechsler were not intended to discriminate reliably much beyond IQ 145, as ceiling effects become a concern.

Since the publication of the Wechsler Adult Intelligence Scale (WAIS), almost all intelligence scales have adopted the normal distribution method of scoring. The use of the normal distribution scoring method makes the term "intelligence quotient" an inaccurate description, mathematically speaking, of the intelligence measurement, but "I.Q." still enjoys colloquial usage, and is used to describe all of the intelligence scales currently in use.

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