Rarely Occurring Headaches and Rarely Occurring Blindness: Is Rarely=Rarely? The Meaning of Verbal Frequentistic Labels in Specific Medical Contexts

In three experimental studies, factors were examined which might influence the interpretation of verbal frequentistic labels in a specific context (i.e. side effects of medical drugs). In Study 1, we directly assessed subjects' numerical equivalents of three verbal labels (recommended for use by the German Federal Health Agency), which were embedded in stylized leaflets for medical drugs; we compared the estimates with those obtained in a context-free situation. The major findings were that (1) subjects' numerical equivalents for the verbal labels were clearly lower in the context condition than in the context-free situation, (2) subjects'; interpretations appeared to depend on the seriousness of the side effects, but (3) this dependency disappeared when explicit base rates were provided. In Studies 2 and 3, we indirectly examined the influence of the seriousness of the side effects, their frequency, and of the mode of expression (verbally versus numerically) by asking for risk judgments for and choices between drugs. Results of particular importance were that (1) seriousness and frequency of side effects had the same influence on judgments and choices but the mode of expression had no significant effect on either judgments or choices; (2) when subjects’ individual numerical equivalents of verbal labels were used in the leaflets, numerically described drugs were judged equally risky as verbally described drugs but were significantly preferred for consumption. These findings have theoretical relevance for research on the processing of verbal and numerical frequentistic expressions as well as a number of practical implications for the communication of medical information.