How Should Doctors Frame the Risk of a Vaccine's Adverse Side Effects? It Depends on How Trustworthy They Are.

BACKGROUND: How health workers frame their communication about vaccines' probability of adverse side effects could play an important role in people's intentions to be vaccinated (e.g., positive frame: side effects are unlikely v. negative frame: there is a chance of side effects). Based on the pragmatic account of framing as implicit advice, we expected that participants would report greater vaccination intentions when a trustworthy physician framed the risks positively (v. negatively), but we expected this effect would be reduced or reversed when the physician was untrustworthy. DESIGN: In 4 online experiments (n = 191, snowball sampling and n = 453, 451, and 464 UK residents via Prolific; Mage≈ 34 y, 70% women, 84% White British), we manipulated the trustworthiness of a physician and how they framed the risk of adverse side effects in a scenario (i.e., a chance v. unlikely adverse side effects). Participants reported their vaccination intention, their level of distrust in health care systems, and COVID-19 conspiracy beliefs. RESULTS: Physicians who were trustworthy (v. untrustworthy) consistently led to an increase in vaccination intention, but the way they described adverse side effects mattered too. A positive framing of the risks given by a trustworthy physician consistently led to increased vaccination intention relative to a negative framing, but framing had no effect or the opposite effect when given by an untrustworthy physician. The exception to this trend occurred in unvaccinated individuals in experiment 3, following serious concerns about one of the COVID vaccines. In that study, unvaccinated participants responded more favorably to the negative framing of the trustworthy physician. CONCLUSIONS: Trusted sources should use positive framing to foster vaccination acceptance. However, in a situation of heightened fears, a negative framing-attracting more attention to the risks-might be more effective. HIGHLIGHTS: How health workers frame their communication about a vaccine's probability of adverse side effects plays an important role in people's intentions to be vaccinated.In 4 experiments, we manipulated the trustworthiness of a physician and how the physician framed the risk of adverse side effects of a COVID vaccine.Positive framing given by a trustworthy physician promoted vaccination intention but had null effect or did backfire when given by an untrustworthy physician.The effect occurred over and above participants' attitude toward the health care system, risk perceptions, and beliefs in COVID misinformation.

People prefer to predict middle, most likely quantitative outcomes (not extreme ones), but they still over-estimate their likelihood.

Past work showed a tendency to associate verbal probabilities (e.g., possible, unlikely) with extreme quantitative outcomes, and to over-estimate the outcomes' probability of occurrence. In the first four experiments (Experiment 1, Experiments 2a-c), we tested whether this "extremity effect" reflects a general preference for extreme (vs central or less extreme) values of a distribution. Participants made predictions based on a frequency distribution in two scenarios. We did not find a preference for extreme outcomes. Instead, most of the participants made a prediction about the middle, most frequent outcome of the distribution (i.e., the modal outcome), but still over-estimated the outcomes' probabilities. In Experiment 3, we tested whether the over-estimation could be better explained by an "at least"/"at most" reading of the predictions. We found that only a minority of participants interpreted predictions as the lower/upper bounds of an open interval and that these interpretations were not associated with heightened probability estimates. In the final three experiments (Experiments 4a-c), we tested whether participants perceived extreme outcome predictions as more correct, useful and interesting than modal outcome predictions. We found that extreme and modal predictions were considered equally correct, but modal predictions were judged most useful, whereas extreme predictions were judged to be more interesting. Overall, our results indicate that the preference for extreme outcomes is limited to specific verbal probability expressions, whereas the over-estimation of the probability of quantitative outcomes may be more general than anticipated and applies to non-extreme values as well.

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.

The Interpretation of Verbal Probabilities: A Systematic Literature Review and Meta-Analysis.

INTRODUCTION: Verbal probabilities such as "likely" or "probable" are commonly used to describe situations of uncertainty or risk and are easy and natural to most people. Numerous studies are devoted to the translation of verbal probability expressions to numerical probabilities. METHODS: The present work aims to summarize existing research on the numerical interpretation of verbal probabilities. This was accomplished by means of a systematic literature review and meta-analysis conducted alongside the MOOSE-guidelines for meta-analysis of observational studies in epidemiology. Studies were included, if they provided empirical assignments of verbal probabilities to numerical values. RESULTS: The literature search identified 181 publications and finally led to 21 included articles and the procession of 35 verbal probability expressions. Sample size of the studies ranged from 11 to 683 participants and covered a period of half a century from 1967 to 2018. In half of the studies, verbal probabilities were delivered in a neutral context followed by a medical context. Mean values of the verbal probabilities range from 7.24% for the term "impossible" up to 94.79% for the term "definite". DISCUSSION: According to the results, there is a common 'across-study' consensus of 35 probability expressions for describing different degrees of probability, whose numerical interpretation follows a linear course. However, heterogeneity of studies was considerably high and should be considered as a limiting factor.

Communicating uncertainty using words and numbers.

Life in an increasingly information-rich but highly uncertain world calls for an effective means of communicating uncertainty to a range of audiences. Senders prefer to convey uncertainty using verbal (e.g., likely) rather than numeric (e.g., 75% chance) probabilities, even in consequential domains, such as climate science. However, verbal probabilities can convey something other than uncertainty, and senders may exploit this. For instance, senders can maintain credibility after making erroneous predictions. While verbal probabilities afford ease of expression, they can be easily misunderstood, and the potential for miscommunication is not effectively mitigated by assigning (imprecise) numeric probabilities to words. When making consequential decisions, recipients prefer (precise) numeric probabilities.

Verbal Probabilities: Linear or Logistic? - A Regression Analysis Approach.

Verbal probability expressions are quite intuitive and used in a variety of clinical important issues like adverse drug causality appraisal. However, there is insufficient evidence of their numerical meaning and whether they have a linear or logistic relationship with them. We aimed at contributing to answering these questions by means of a comparative regression analysis based on a sample of N=683 participants between 10 and 82 years (mean age 20.33±11.77; median: 18 years) who were asked to numerically rate a given set of sixteen verbal probability phrases on a visual analogue scale. With respect to the explained variance, we found an R2 for the linear model of 0.574 while R2 for the logistic model was only 0.392 indicating a superiority of linear model compared to the logistic model. Although we were able to show that ranked verbal phrases are more likely to behave in a linear that in a logistic way other regression options like the double logistic model should be taken into consideration for further research.

Communicative functions of directional verbal probabilities: Speaker's choice, listener's inference, and reference points.

Verbal probabilities have directional communicative functions, and most can be categorized as positive (e.g., "it is likely") or negative (e.g., "it is doubtful"). We examined the communicative functions of verbal probabilities based on the reference point hypothesis According to this hypothesis, listeners are sensitive to and can infer a speaker's reference points based on the speaker's selected directionality. In four experiments (two of which examined speakers' choice of directionality and two of which examined listeners' inferences about a speaker's reference point), we found that listeners could make inferences about speakers' reference points based on the stated directionality of verbal probability. Thus, the directionality of verbal probabilities serves the communicative function of conveying information about a speaker's reference point.