On the terminology and dimensions of incidence

This paper is a review of the concept of incidence and the measures of incidence commonly used. The three components of incidence (observed event, population at risk, and observation period) and the four measures of incidence (incidence as number of events, as number of events per time unit, as number of events per unit of amounts of observation, and as probability) are discussed. The terminology and dimensions of incidence measures used in epidemiology literature are reviewed. Ambiguities and uses of the same term for different purposes seem to suggest the need for standardized terminology. Incidence measures with length-based amount of observation are also briefly discussed.