More than 50% or Less than 70% Chance: Pragmatic Implications of Single-Bound Probability Estimates

Probability estimates can be given as ranges or uncertainty intervals, where often only one of the interval bounds (lower or upper) is specified. For instance, a weather forecaster can describe La Niña as having “more than 70% chance” or “less than 90% chance” of occurring. In three experiments, we studied how research participants perceived climate related forecasts expressed with lower-bound (“over X% chance”) or upper-bound (“under Y% chance”) probability statements. Results indicate that such single-bound statements give pragmatic information in addition to the numeric probabilities they convey. First, the studies show that these statements are directional, leading the listeners’ attention in opposite directions. “Over” statements guide attention towards the possible occurrence of the event, and are explained by reasons for why it might happen, while “under” statements direct attention to its possible non-occurrence, and are more often explained by reasons for why the target event might not appear, corresponding to positive (it is possible) vs. negative (it is uncertain) verbal probabilities. Second, boundaries were found to reveal the forecaster’s beliefs, and could be perceived as indicative of an increasing or a decreasing trend. Single-bound probability estimates are therefore not neutral communications of probability level, but might “leak” information about the speaker’s expectations, and about past and future developments of the forecast.