The availability heuristic involves judging an event as likely or frequent if instances of it are easy to imagine or recall. This is often appropriate in so far as frequently occurring events are generally easier to imagine or recall than unusual events. However, events may be easily imagined or recalled simply because they have been recently brought to the attention of an individual. Thus a recent incident, recent discussion of a low-probability hazard, or recent media coverage, may all increase memorability and imaginability of similar events and hence perceptions of their perceived likelihood. Conversely, events that an individual has rarely experienced or heard about, or has difficulty imagining, will be perceived as having a low probability of occurrence irrespective of their actual likelihood of occurring. Obviously experience is a key determinant of perceived risk. If experience is biased, then perceptions are likely to be inaccurate.
In some situations, failure to appreciate the limits of presented data may lead to biased probability estimates. For example, Fischoff et al. (1978) studied whether people are sensitive to the completeness of fault trees. They used a fault tree indicating the ways in which a car might fail to start. Groups of subjects were asked to estimate the proportion of failures that might be due to each of seven categories of factors including an 'all other problems' category. When three sections of the diagram were omitted, effectively incorporating removed categories into the 'all other problems' category, subjects overestimated the probability of the remaining categories and substantially underestimated the 'all other problems' category. In effect, what was out of sight was out of mind. Professional mechanics did not do appreciably better on the test than laypeople.
Such findings suggest that fault trees and other representations of sources of uncertainty can strongly influence judgements about probabilities of particular sources occurring. Tables 10.1, 10.2, and 10.3 can be interpreted as a way of exploring the importance of these kinds of issues.
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