Of the different methods that purport to measure intelligence, the most famous is
the IQ (Intelligence Quotient) test, which is a standardized test designed to measure
human intelligence as distinct from attainments.
Intelligence quotient is an age-related measure of intelligence level. The word
quotient means the result of dividing one quantity by another, and one definition of
intelligence is mental ability or quickness of mind.
Usually, IQ tests consist of a graded series of tasks, each of which has been standardized
with a large representative population of individuals in order to establish
an average IQ of 100 for each test.
It is generally accepted that a person’s mental ability develops at a constant rate
until about the age of 13, after which development has been shown to slow down,
and beyond the age of 18 little or no improvement is found.
When the IQ of a child is measured, the subject attempts an IQ test that has been
standardized, with an average score recorded for each age group. Thus a 10-yearold
child who scored the result that would be expected of a 12-year-old would have
an IQ of 120, or 12/10 × 100:
Because after the age of 18 little or no improvement is found, adults have to be
judged on an IQ test whose average score is 100, and the results graded above and
below this norm according to known test scores.
Like so many distributions found in nature, the distribution of IQ takes the form
of a fairly regular bell curve (see Figure 0.1 below) in which the average score is 100
and similar proportions occur both above and below this norm.
There are a number of different types of intelligence tests, for example Cattell,
Stanford-Binet and Wechsler, and each has its own different scales of intelligence.
The Stanford-Binet is heavily weighted with questions involving verbal abilities
and is widely used in the United States. The Weschler scales consist of two separate
verbal and performance sub-scales each with its own IQ rating. On the Stanford-
Binet scale half the population fall between 90 and 110 IQ, half of them above 100
and half of them below; 25 per cent score above 110; 11 per cent above 120; 3 per cent
above 130 and 0.6 per cent above 140. At the other end of the scale the same kind of
proportion occurs.
Although it is IQ tests that we are specifically concerned with in this book it
should be pointed out that IQ tests are just one part of what is generally referred to
as psychometric testing. Such test content may be addressed to almost any aspect of
our intellectual or emotional make-up, including personality, attitude, intelligence
or emotion. Psychometric tests are basically tools used for measuring the mind; the
word metric means measure and the word psycho means mind. There are two types
of psychometric tests that are usually used in tandem by employers. These are
aptitude tests, which assess your abilities, and personality questionnaires, which
assess your character and personality.
