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The representativeness heuristic 253
less reliable. In this case, you are likely to find it easy to recall or
imagine instances of the co-occurrence of the events ‘foreign supplier’
and ‘goods defective’. You find it less easy to recall the many reliable
goods which you have received from foreign suppliers and the defective
ones which you received from domestic suppliers. The result can be
an illusory correlation, that is an overestimation of the frequency with
which the two events occur together. In decision analysis models,
illusory correlation is of concern when conditional probabilities (e.g.
p(goods defective|foreign supplier)) have to be estimated.
In a well-known demonstration of illusory correlation, Chapman and
Chapman 3 conducted an experiment in which naive judges were given
information on several hypothetical mental patients. This information
consisted of a diagnosis and a drawing made by a patient. Later the
judges were asked to estimate how frequently certain characteristics
referred to in the diagnosis, such as suspiciousness, had been accompanied
by features of the drawing, such as peculiar eyes. It was found
that the judges significantly overestimated the frequency with which,
for example, suspiciousness and peculiar eyes had occurred together.
Indeed, this illusory correlation survived even when contradictory evidence
was presented to the judges. This research demonstrates the
powerful and persistent influence which preconceived notions (in this
case that suspiciousness is associated with the eyes) can have on judgments
about relationships.
The representativeness heuristic
Suppose that you are considering the following questions. What is the
probability that Peter, who you met at last night’s conference party,
is a salesman? What is the probability that the defective circuit board,
which you have just been supplied, was manufactured at your Pittsburgh
factory? What is the probability that your sales graph shows
that sales are simply varying randomly, rather than following a cyclical
pattern?
In all of these questions you have to judge the probability that a person
or object belongs to a particular category, or that events originate from
a particular process. If you use the representativeness heuristic you
will answer these questions by judging how representative the object,
person or event is of the category or process. For example, Peter was a
street-wise extrovert who talked quickly and wore smart clothes. If you