The intelligence quotient represents a measurement concept that was used extensively in the early days of intelligence testing but is less commonly used today. After Alfred Binet’s death in 1911, (Stern) introduced the notion of a mental quotient, suggesting that the index of intellectual functioning derived from the Binet-Simon Scale could be expressed as the ratio of a test taker’s mental age to his or her chronological age multiplied by 100 to eliminate decimals (MQ = 100 × MA/CA). This MQ represented something about a person’s rate of mental growth up to the time of the test. If examinees earned a mental age (MA) equivalent to chronological age (CA), their mental quotient (MQ) would be 100. An MQ of 100 represented average performance.
Working at Stanford University in California, Lewis M. Terman developed what was to become the most widely used American version of the Binet test, the Stanford-Binet. (Terman) incorporated Stern’s notion of a mental quotient but renamed it, calling it a ratio intelligence quotient, or IQ.
The concept of the ratio intelligence quotient became increasingly popular, but it was used in a number of inappropriate ways. Its decline over the past quarter century can be attributed to a number of inherent characteristics that have been highly criticized by measurement specialists and practitioners.
Because the ratio intelligence quotient has minor differences in the magnitude of its standard deviation at various ages, a constant intelligence quotient from one age to another does not represent the same relative status. Similarly, even if the test taker’s relative status remained the same from one year to another, the intelligence quotient would have to change. This suggests that intelligence quotients at different age levels are not comparable statistically (Tyler & Walsh,). For example, a very bright child could obtain a higher IQ at age 12 than at age 6, even if the child’s growth rate was unchanged. This difference would simply be due to the differences in the variability or standard deviations, with the variability of the IQ distribution being greater for 12-year-olds than for 6-year-olds.
Critics also point to the conceptual difficulty of the ratio intelligence quotient. For example, a 5-year-old with a mental age of 6 and a 10-year-old with a mental age of 12 would both have identical intelligence quotients of 120. However, the 6-year-old is a year advanced in mental age while the 10-year-old is 2 years advanced.
Another criticism of the intelligence quotient relates to its inability to describe adult intelligence (Tyler & Walsh,). Critics suggest that like physical growth, adult mental growth lacks the predictable regularity characteristic of the mental development of children. Age standards lack meaning after the mid-teens, rendering mental age and therefore the intelligence quotient concept meaningless.
Owing to these criticisms of the ratio intelligence quotient, major intelligence tests in use today yield IQs but not ratio intelligence quotients. Because of the inherent limitations of the ratio intelligence quotient, they should be interpreted cautiously.
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