those that are unfamiliar or hypothetical (Slovic et al., 1979; Lichtenstein et al., 1978; Fischhoff et al., 1978). Morgan (1993) uses observability and controllability as the two dimensions that characterize a hazard's "dreadfulness" and the degree to which it is understood (see Figure 1).

Heuristics and Biases

Cognitive shortcuts or rules of thumb known as heuristics affect peoples' quantitative estimates of risk. Risk scientists have shown that there are regular and predictable patterns in the ways that these operate. Use of these heuristics can result in biases in quantitative estimates of risk.

Anchoring refers to a lack of feel for absolute frequency and a tendency for people to estimate frequencies for a new event on the basis of the frequencies presented for other events. For example, if a person is told that 1,000 people a year die from electrocution and then is asked to estimate how many people die from influenza, his or her number is likely to be lower than if the person is first told that 45,000 people a year die in automobile accidents (Kahneman and Tversky, 1972). The tendency is to "anchor" on the first number and not adjust far enough from it. Consequently, how and what probability estimates of risk are presented and in what order they are presented may affect how risks are perceived because of anchoring effects.

Compression is the overestimation of small frequency risks and the underestimation of large frequency risks (Fischhoff et al., 1993). If this applied to vaccine risks, people would behave as if the risk of rare adverse effects from vaccines were higher than reported.

Availability means that events that are easily remembered or imagined are more accessible or "available" to people, so that their frequencies are overestimated (Tversky and Kahneman, 1973). If, for example, a particular risk has recently or often been reported in the popular press, people may well overestimate its frequency. A science writer commented that people pay more attention to dramatic, new, or unknown risks or risks conveyed within the context of a personal story. Most people will give proportionally more weight to a dramatic risk of dying from an airplane crash, for example, than to the risk of dying from lung cancer due to smoking, even though the latter is more likely. Drama, symbolism and identifiable victims, particularly children or celebrities, the science writer said, also make a risk more memorable.

When risks are given as verbal probabilities (e.g., likely, unlikely, rare, and common), interpretation depends on the context (Budescu and Wallsten, 1985; Wallsten et al., 1986). The phrase "likely to catch a cold" will be interpreted differently from "likely to become infected with HIV," for example.

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