Do claims about certainty make estimates less certain?

Predictions and estimates are sometimes qualified as certain. This epistemic marker occupies a privileged position at the top of scales of verbal probability expressions, reflecting probabilities close to 1. But such statements have rarely been compared to plain, unqualified statements in which certainty is not mentioned. We examined in nine studies (N = 2784) whether statements explicitly claimed to be certain are perceived as (1) more (or less) credible, (2) more (or less) precise, and (3) more (or less) strongly based upon evidence, compared to plain, unmarked declarative statements. We find, in apparent contrast with assumptions made by the standard scales, that "certain" are often judged to be less trustworthy, less reliable, and held with lower confidence than unmarked statements. Plain, declarative statements are further assumed to be more precise, while certainty implies that more extreme outcomes are possible. When it is certain that Henry made four errors, it is clear he did not commit less than four, but he might have committed five errors or more. Thus certainty can indicate lower bounds of an interval whose upper bounds are not defined, and certainty statements are consequently more ambiguous than estimates that do not mention certainty. At least-interpretations of certainty affect the interpretation of options in risky choice problems, where "200 lives will be saved" was deemed by a majority to mean exactly 200, while "it is certain that 200 will be saved", could mean 200-600 lives. We also find that credibility is affected by type of certainty, with impersonal certainty ("it is certain") perceived to be more accurate and persuasive than personal certainty ("I am certain"), especially in predictions of future events. Moreover, mentions of certainty can reveal that that a speaker's estimate is based on subjective judgments and guesswork rather than upon objective evidence. These findings have implications for communication. Estimates can appear more consensual when claims of certainty are omitted. To convey certainty it may be better not to mention that one is certain.

Dimensions of uncertainty communication: What is conveyed by verbal terms and numeric ranges

The paper reviews two strands of research on communication of uncertainty that usually have been investigated separately: (1) Probabilities attached to specific outcomes, and (2) Range judgments. Probabilities are sometimes expressed by verbal phrases ("rain is likely") and at other times in a numeric format ("70% chance of rain"), whereas range judgments describe the potential amounts expected ("1-4 mm of rain"). Examination of previous research shows that both descriptions convey, in addition to the strength of expectations, pragmatic information about the communicative situation. For instance, so-called verbal probability expressions (VPE), as likely, unlikely, a chance, or not certain give some, albeit vague, probabilistic information, but carry in addition an implicit message about the sources of uncertainty, the outcome's valence and severity, along with information about the speakers' attitudes and their communicative intentions. VPEs are directional by drawing attention either to an outcome's occurrence ("it is possible") or to its non-occurrence ("it is doubtful"). In this sense they may be more informative than numbers. Uncertainties about outcomes in a distribution (continuous quantities) are alternatively expressed as interval estimates. The width of such intervals can function as a cue to credibility and expertise. Incomplete, one-sided intervals, where only one boundary is stated, imply directionality. "More than 100 people" suggests a crowd, while "less than 200" implies a shortfall. As with VPEs, directionally positive intervals are more frequent, and perhaps more neutral than negative ones. To convey expectancies and uncertainty in a balanced way, communicators may have to alternate between complementary frames.

More is easier? Testing the role of fluency in the more-credible effect

People are more likely to endorse statements of the form "A is more than B" than those of the form "B is less than A", even though the ordinal relationship being described is identical in both cases -– a result I dub the "more-credible" effect. This paper reports 9 experiments (total N = 5643) that probe the generality and basis for this effect. Studies 1–4 replicate the effect for comparative statements relating to environmental change and sustainable behaviours, finding that it is robust to changes in participant population, experimental design, response formats and data analysis strategy. However, it does not generalize to all stimulus sets. Studies 5–9 test the proposition that the effect is based on the greater ease of processing "more than" statements. I find no meaningful effect of warning people not to base their judgments on the fluency of the sentences (Studies 5 and 6), but do find associations between comparative language, credibility, and processing time: when the more-credible effect manifests, the more-than statements are read more quickly than the less-than statements, and this difference partly mediates the effect of comparative on agreement with the statements; in contrast, for a set of comparisons for which changes in the more/less framing did not affect truth judgments, there was no meaningful difference in the time taken to read the more- and less-than versions of the statements. Taken together, these results highlight the importance of comparative language in shaping the credibility of important socio-political messages, and provide some limited support for the idea that the effect of language choice is partly due to differences in how easily the statements can be processed -– although other mechanisms are also likely to be at work.

Guidance on Communication of Uncertainty in Scientific Assessments

This document provides guidance for communicators on how to communicate the various expressions of uncertainty described in EFSA's document: 'Guidance on uncertainty analysis in scientific assessments'. It also contains specific guidance for assessors on how best to report the various expressions of uncertainty. The document provides a template for identifying expressions of uncertainty in scientific assessments and locating the specific guidance for each expression. The guidance is structured according to EFSA's three broadly defined categories of target audience: 'entry', 'informed' and 'technical' levels. Communicators should use the guidance for entry and informed audiences, while assessors should use the guidance for the technical level. The guidance was formulated using evidence from the scientific literature, grey literature and two EFSA research studies, or based on judgement and reasoning where evidence was incomplete or missing. The limitations of the evidence sources inform the recommendations for further research on uncertainty communication.

Is guilt 'likely' or 'not certain'? Contrast with previous probabilities determines choice of verbal terms

This research focuses on what determines speakers' choice of positive and negative probability phrases (e.g., "a chance" vs. "not certain") in a legal context. We argue that choice of phrase to describe an event's probability of occurrence can be determined by the contrast between its current p value and an earlier p value, and not by that current value alone. Three experiments were conducted describing scenarios where profilers communicated a suspect's probability of guilt to the police. In the first study, a probability estimate is revised upwards or downwards. In the second one, the probability estimate of a speaker is higher or lower than that given by a previous speaker. In both cases, participants expected upward trends to lead to positive phrases, whereas downward trends were associated with negative phrases. In a third study, participants had to select probability phrases to characterize two different suspects. No contrast effects were found. We conclude that verbal probability directionality has primarily an argumentative function, where positive phrases are selected when probabilities are contrasted with smaller p values, and negative when contrasted with higher p values.

Maximum vs. minimum values: preferences of speakers and listeners for upper and lower limit estimates

Estimates about uncertain quantities can be expressed in terms of lower limits (more than X, minimum X), or upper limits (less than Y, maximum Y). It has been shown that lower limit statements generally occur much more often than upper limit statements (Halberg & Teigen, 2009). However, in a conversational context, preferences for upper and lower limit statements will be moderated by the concerns of the interlocutors. We report three studies asking speakers and listeners about their preferences for lower and upper limit statements, in the domains of distances, durations, and prices. It appears that travellers prefer information about maximum distances and maximum durations, and buyers (but not sellers) prefer to be told about maximum prices and maximum delivery times. Mistaken maxima are at the same time regarded as more "wrong" than mistaken minima. However, this preference for "worst case" information is not necessarily shared by providers of information (advisors), who are also concerned about being blamed if wrong.