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.
Keywords: Meta-analysis; Numerical representation; Systematic review; Verbal probabilities.