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252 Biases in probability assessment
people’s estimates of the probability of death by various causes were
moderately accurate, there were some serious misperceptions. These
were closely associated with recent reports of deaths by these causes in
newspapers. For example, the probabilities of death by animal bites and
stings were grossly overestimated. Similarly, accidents and disease were
thought to be equally likely causes of death. In reality, diseases cause 15
times more deaths than accidents.
2. Ease of imagination is not related to probability
Easily imagined events are not necessarily highly probable. The civil
engineer in charge of a construction project may find it easy to imagine
all of the circumstances which could lead to the delay of the project
such as strikes, adverse weather conditions, geological problems and
interruptions in the supply of materials and equipment. The result
could be a gross overestimate of the risk of the project overrunning the
deadline. Conversely, risks may sometimes be underestimated because
the dangers associated with a course of action are difficult to imagine.
Test your judgment: answers to questions 1–3
Were your probability estimates in the questionnaire distorted by ease
of recall or ease of imagination?
Q1. Approximately 1 in 500 people were victims of robbery in the USA
in 1991, according to the official statistics.
Q2. In Britain, driving a car for an hour is seven times more likely to
lead to fatal injury.
Q3. (a) In the USA strokes cause 1 in 14 deaths, while accidents cause 1
in 23.
(b) Influenza is a far more common cause of death than aeroplane
crashes. It kills about 70 000 people per year.
3. Illusory correlation
Suppose that you are a manager of a factory and you are considering
whether you are more likely to obtain defective goods from a domestic
or foreign supplier. Before thinking about your answer you may already
have some preconceptions, for example that foreign goods tend to be