This thesis investigates the effect of communication format on the understanding of uncertainty communications and considers the implications of these findings for a communicator’s perceived credibility. The research compares five formats: verbal probability expressions (VPEs; e.g., ‘unlikely’); numerical expressions – point (e.g., ‘20% likelihood’) and range estimates (e.g., ‘10–30% likelihood’); and mixed expressions in two orders (verbal-numerical, e.g., ‘unlikely [20% likelihood]’ and numerical-verbal format, e.g., ‘20% likelihood [unlikely]’). Using the ‘which-outcome’ methodology, we observe that when participants are asked to estimate the probability of the outcome of a natural hazard that is described as ‘unlikely’, the majority indicate outcomes with a value exceeding the maximum value shown, equivalent to a 0% probability. Extending this work to numerical and mixed formats, we find that 0% interpretations are also given to communications using a verbal-numerical format (Chapter 2). If ‘unlikely’ is interpreted as referring to events which will never occur, there could be implications for a communicator’s perceived credibility should an ‘unlikely’ event actually occur. In the low probability domain, we find a communicator who uses a verbal format in their prediction is perceived as less credible and less correct than one who uses a numerical format. However, in the high probability domain (where a ‘likely’ event does not occur) such an effect of format is not consistently observed (Chapter 3). We suggest ‘directionality–outcome congruence’ can explain these findings. For example, the negatively directional term ‘unlikely’ led to harsher ratings because the outcome was counter to the original focus of the prediction (i.e., on its non-occurrence). Comparing communications featuring positively and negatively directional VPEs, we find that communicators are perceived as less credible and less correct given directionality–outcome incongruence (Chapter 4). Our findings demonstrate the influence of pragmatics on (a) the understanding of uncertainty communications and (b) perceived communicator credibility
