Incidence quantifies the number of new cases of a diseases or events occurring in a specified time period, often a year, to a defined population who are at risk of the disease/event. It is given as a rate. Cumulative incidence describes the proportion of those at risk who develop the disease, or experience an event, over a specified period of time (often aggregated over a number of years). Incidence density, sometimes called force of morbidity or mortality, is a more precise concept in which those who develop a disease are removed from the eligible population as they are no longer eligible to develop the disease. So the denominator for incidence density becomes the aggregated person-time of eligibility (‘person-years of exposure’), rather than the number of individuals eligible at the start. Specific types of incidence rate are mortality rates (deaths in population), morbidity (non-fatal disease in population), case fatality rates (deaths in diseased population) and attack rate (cases of disease in a population at risk, usually over a short period of observation). Comparisons of incidence rates may be misleading if there are differences in risk factors (for example, the age distribution) between the populations compared. In this case various techniques such as standardisation (often used to compare mortality), or regression may be used to adjust for these known differences.

How to cite: Incidence [online]. (2016). York; York Health Economics Consortium; 2016. https://www.yhec.co.uk/glossary/incidence/

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