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The availability heuristic 251
maker, they can also lead to systematically biased judgments. Three
main heuristics identified by Tversky and Kahneman are (i) availability,
(ii) representativeness and (iii) anchoring and adjustment. We consider
these, and their associated biases, next.
The availability heuristic
Suppose that you are asked to assess the probability that a retail business
will fail within the next year. If you use the availability heuristic, you
will search your mind in an attempt to recall examples of shops failing
or you will try to imagine circumstances which will lead to the demise
of the business. If instances of shops failing are easily recalled, or the
circumstances leading to closure are easily imagined, then you will
judge the probability of failure to be high. People using the availability
heuristic therefore judge the probability of the occurrence of events by
how easily these events are brought to mind. Events which are vivid,
recent, unusual or highlighted by the media are readily recalled or
envisaged and therefore assigned high probabilities. Events which are
less available to the mind are assumed to be less likely.
Availability can be a reliable heuristic. Frequently occurring events
are usually easier to recall so the higher probabilities estimates for
them should be reliable. However, the ease with which an event can
be recalled or imagined sometimes has no relationship to the true
probability of the event occurring. For example, some events are easily
recalled precisely because they are unusual and rare. This can lead to
biased estimates. Some of the biases associated with the availability
heuristic are discussed next.
Biases associated with the availability heuristic
1. When ease of recall is not associated with probability
Easily recalled events are not necessarily highly probable. In Britain a
study by the Association of Chief Police Officers in 1996 found that
elderly people were rarely victims of violent crime. Yet the perception of
elderly people was that the risk of their being attacked was frighteningly
high. Although rare, when they do occur, attacks on old people are
headline news and instances of such crimes are therefore easily and
vividly recalled. Similarly, studies 2 in the USA found that, although