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What is IQ

The concept of the Intelligence Quotient (IQ) was developed by Stern (1912), from Binet’s notion of mental age. Stern defined IQ as mental age divided by chronological age multiplied by 100. Previous to Stern’s work, chronological age had been subtracted from mental age to provide a measure of mental alertness. Stern on the other hand showed that it was more appropriate to take the ratio of these two constructs, to provide a measure of the child’s intellectual development that was independent of the child’s age. He further proposed that this ratio should be multiplied by 100 for ease of interpretation; thus avoiding cumbersome decimals.

Clearly, the notion of obtaining a person’s IQ by dividing their mental age by their chronological age only makes intuitive sense for children. The concept of IQ was therefore subsequently redefined in terms of the person’s position on a distribution of test scores of people of similar age. To maintain continuity with Stern’s definition of IQ, the IQ scale is defined as having a mean of 100, so that in line with Stern an IQ of 100 indicates an average level of mental ability, or general intelligence. The IQ scale was further defined as having a standard deviation of 15. To help you understand what this means in practice, the definition given to each band of IQ scores by the WAIS manual is noted in the table below.

IQ Range	Definition	Percentile Rank
130 +	Very Superior	98%
120-129	Superior	91%
110-119	High Average	75%
90-109	Average	50%
80-89	Low Average	25%
70-79	Borderline	8%
69 -	Learning Difficulties	< 2%

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