The Use Of The Intelligence Quotient

As elsewhere explained, the mental age alone does not tell us what we want to know

about a child's intelligence status. The significance of a given number of years of

retardation or acceleration depends upon the age of the child. A

3-year-old child who is retarded one year is ordinarily feeble-minded; a

10-year-old retarded one year is only a little below normal. The child

who at 3 years of age is retarded one year will probably b
retarded two

years at the age of 6, three years at the age of 9, and four years at

the age of 12.

What we want to know, therefore, is the ratio existing between mental

age and real age. This is the intelligence quotient, or I Q. To find it

we simply divide mental age (expressed in years and months) by real age

(also expressed in years and months). The process is easier if we

express each age in terms of months alone before dividing. The division

can, of course, be performed almost instantaneously and with much less

danger of error by the use of a slide rule or a division table. One who

has to calculate many intelligence quotients should by all means use

some kind of mechanical help.