Understanding Psychology (10th Ed)

mental age
The age for which a given 
level of performance is average or 
typical.
intelligence quotient (IQ)
A score 
that takes into account an individual’s 
mental and chronological ages.
Now in its fi fth edition, the Stanford-Binet 
test consists of a series of items that vary 
in nature according to the age of the 
person being tested. What can we learn 
about a person from a test of this type?
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Module 26 
What Is Intelligence? 
289
As a bit of trial and error with the formula will show you, anyone who has a 
mental age equal to his or her chronological age will have an IQ equal to 100. More-
over, people with a mental age that is lower than their chronological age will have 
IQs that exceed 100. 
Although the basic principles behind the calculation of an IQ score still hold, 
today IQ scores are determined in a different manner and are known as deviation IQ 
scores. First, the average test score for everyone of the same age who takes the test 
is determined, and that average score is assigned an IQ of 100. Then, with the aid of 
statistical techniques that calculate the differences (or “deviations”) between each 
score and the average, IQ scores are assigned. 
As you can see in Figure 5, when IQ scores from large numbers of people are 
plotted on a graph, they form a bell-shaped distribution (called “bell-shaped” because 
it looks like a bell when plotted). Approximately two-thirds of all individuals fall 
within 15 IQ points of the average score of 100. As scores increase or fall beyond that 
range, the percentage of people in a category falls considerably.