intelligence quotient

The intelligence quotient of this girl is 140, which is not reached by more than one child in two hundred.

Thus, an error of 10 months in the mental age of a 5-year-old means an error of almost 15 per cent in the intelligence quotient.

Stating the error in terms of the intelligence quotient makes it more evident.

The intelligence quotient (often designated as I Q) is the ratio of mental age to chronological age.