IGENCE 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 be 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.

HOW TO FIND THE I Q OF ADULT SUBJECTS. Native intelligence, in so far as
it can be measured by tests now available, appears to improve but little
after the age of 15 or 16 years. It follows that in calculating the I Q
of an adult subject, it will be necessary to disregard the years he has
lived beyond the point where intelligence attains its final development.

Although the location of this point is not exactly known, it will be
sufficiently accurate for our purpose to assume its location at
16 years. Accordingly, any person over 16 years of age, however old, is
for purposes of calculating I Q considered to be just 16 years old. If a
youth of 18 and a man of 60 years both have a mental age of 12 years,
the I Q in each case is 12 / 16, or .75.

The significance of various values of the I Q is set forth
elsewhere.[44] Here it need only be repeated that 100 I Q means exactly
average intelligence; that nearly all who are below 70 or 75 I Q are
feeble-minded; and that the child of 125 I Q is about as much above the
average as the high-grade feeble-minded individual is below the average.
For ordinary purposes all who fall between 95 and 105 I Q may be
considered as average in intelligence.

[44] See Chapter VI.

MATERIAL FOR USE IN TESTING. It is strongly recommended that in testing
by the Stan