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.