This is the ratio between the mental age and chronological age multiplied by 100. Thus, if a boy of 10 years of age is found to have a mental age of 12 years, his IQ will be 120. On the other hand, if he is found to have a mental age of 8 years, his IQ will be 80.
The mental age is established by various tests, the most widely used of which are the Stanford-Binet Scale, the Wechsler Adult Intelligence Scale, and the Mill Hill Vocabulary Test.
Average intelligence is represented by an IQ of 100, with a range of 85 to 115. For practical purposes it is taken that the intellectual level reached by the average 15-year-old is indistinguishable from that of an adult.