There is a 60% probability, but I am 70% certain: communicative consequences of external and internal expressions of uncertainty

Abstract

ABSTRACTCurrent theories of probability recognise a distinction between external (un)certainty (frequentistic probabilities) and internal (un)certainty (degrees of belief). The present studies investigated this distinction in lay people's judgements of probability statements formulated to suggest either an internal (“I am X% certain”) or an external (“It is X% certain” or “There is an X% probability”) interpretation. These subtle differences in wording influenced participants' perceptions and endorsements of such statements, and their impressions of the speaker. External expressions were seen to signal more reliable task duration estimates, and a lower degree of external than internal certainty was deemed necessary to advise a course of action. In conversations about football, internal expressions were perceived as signalling more personal interest, and were expected to be on the average 10% higher than corresponding external probabilities. Finally, people who reported their outcome expectations for two major sports events let their degree of interest in these events influence their internal but not their external certainty. These results have implications for the communication of uncertainty and probability.