Words or numbers? The evaluation of probability expressions in general practice

A sample of 56 general practitioners were asked to rate, on a percentage scale, 23 words or phrases which denote frequency or likelihood. The hypothetical context of the exercise was that of communicating to patients the probability of a side-effect (headache) arising from an unspecified prescription medicine. Median phrase ratings ranged from 'never' at 0% to 'certain' at 95% with a 50% rating given to the phrase 'reasonable chance'. Despite relatively large variance in ratings between respondents, the median ratings of a number of phrases were similar, and some identical, to other studies from different medical professionals. Although the clinical context in which a given expression of probability is used may affect its meaning, the results are encouraging and suggest that phrases denoting likelihood might be systematically codified to enhance communication between doctor and patient. To move towards this objective more research is needed to evaluate how patients interpret expressions of probability, and the relative effectiveness of different modes of communicating likelihood.