When I use a word . . . Medical powers of three

The number three is a powerful guide to formulating slogans, statements of principles, and story telling in all walks of life, including medicine. The statistical rule of three states that the 95 confidence interval of a zero observation is 0–3, so that if you don’t see a particular outcome, such as an adverse reaction, in a clinical trial of an intervention, there is still a chance that it will occur when the intervention is used more widely; thus, if an event is not seen in a study of 100 individuals there is up to a 3% chance of seeing it in practice; if it isn’t seen in 1000 individuals the chance of seeing it subsequently is up to 0.3% and so on. Triadic medical slogans include “see one, do one, teach one”; “history, examination, investigation”; and “percussion, palpation, auscultation.” There is a rule of three that has been applied to fragment-based drug discovery, and the triad that informs subsequent drug development is “quality, efficacy, safety,” although not necessarily in that order. Most clinical syndromes have at least three components, although there are a few with only one or two.