Engler, D. (1958). What is the IQ? In D. Engler, How to raise your child's IQ (pp. 27-32). New York, NY, US: Criterion Books.
http://dx.doi.org/10.1037/14033-004
It is generally known that the IQ is a number, that the number measures level of intelligence, and that the level of intelligence is determined by the individual's performance on an intelligence test. Beyond that the average person knows little about the IQ. The letters IQ stand, of course, for Intelligence Quotient. This quotient is obtained by dividing mental age by chronological age and multiplying the result by 100. It is Mental Age that is supposedly measured by the test. For example, if a child's test score indicates a mental age of nine years and the child is actually aged eight years and six months, his IQ would be 106. (PsycINFO Database Record (c) 2016 APA, all rights reserved)