The most widely used formal language of uncertainty in risk estimates is probability1 (Morgan, 2009). As Spiegelhalter et al. (2011) stated, however, “probabilities are notoriously difficult to communicate effectively to lay audiences.” Probabilistic information, and the uncertainties associated with those probabilities, can be communicated using numeric, verbal, or graphic formats, and consideration should be given to which approach is most appropriate. In a recent review Spiegelhalter et al. (2011) pointed out that the available research in this area for the most part is limited to small studies, often on students or self-selected samples. That lack of large, randomized experiments remains years after Bostrom and Löfstedt (2003) “concluded that risk communication was ‘still more art than science, relying as it often does in practice on good intuition rather than well-researched principles’” (Spiegelhalter et al., 2011, p. 1399).
As discussed later in this chapter, the most appropriate approach to communicating uncertainty depends on the circumstances (Fagerlin et al., 2007; Nelson et al., 2009; Spiegelhalter et al., 2011; Visschers et al., 2009). Lipkus (2007) summarized the general strengths and weaknesses of each of the different approaches for conveying probabilistic information, based on a comprehensive literature review and consultation with risk communication experts (see Box 6-1). The committee discusses relevant findings from this research below. Regardless of the format in which the uncertainty is presented, it is important to bound the uncertainty and to describe the effect it might have on a regulatory decision. Presenting the results of analyses such as the sensitivity analyses and scenarios discussed in Chapter 5 is one way to provide some boundaries on the effects of those uncertainties and to educate stakeholders about how those uncertainties might affect a decision. It is important to note that the existence of weaknesses does not necessarily indicate that a given method should not be used, but rather those weaknesses should be considered and adjusted for when developing a communication strategy.
Numeric Presentation of Uncertainty
In general, numeric presentations of probabilistic information—such as presenting information in terms of percentages and frequencies—can lead to more accurate perceptions of risk than verbal and graphic formats (Budescu et al., 2009). Unlike graphic and verbal presentations, numeric information can be put into tables in order to communicate a large amount of information in a single presentation. For example, Table 6-1, created by EPA,
1 Probability is a form of uncertainty information.