How does contextual information affect our understanding of numerical probabilities

Abstract

Numerical probabilities representing health risks or treatment benefits are constantly communicated to the general public. These probabilities are often embedded in contextual information in the form of a personal patient narrative or newscasts. Previous research suggest that people’s perceptions of risk will be greater when probability information is communicated along with contextual information. In this study, we hypothesized that the presence of contextual information, in the form of a televised newscast, would increase the participant’s perception of risk of side effects of the swine flu vaccine. Participants (N = 75) were presented with information about the side effects of the swine flu vaccine. Approximately half the Norwegian population chose to get vaccinated in 2009. We explored how the presence or absence of contextual information and numerical probabilities affected the perception of risk. The participants reported substantially increased risk perceptions when contextual information was present. In its absence, participants reported relatively low perceptions of risk. These results clearly indicate a potential gap in how probability information is presented and the recipient’s assessment and perception of it. The under- or overestimation of probability information has important implications for medical decision-making.