Cumulative incidence

Cumulative incidence (sometimes referred to as incidence proportion) is a measure of disease frequency which counts the proportion of a candidate population that becomes diseased/develops disease over a specified period of time. Cumulative incidence measures occurrence of new cases of disease. It involves the transition from one state to another, such as non-disease to diseased state (incidence of disease) or diseased to non-diseased (incidence of cure).

Specific forms of cumulative incidence include:

• Crude mortality rate: number of deaths due to any cause/100,000 population during a specified time period
• Infant mortality rate: number of infant deaths due to any cause/1,000 population during a specified time period
• Morbidity rate: number of illnesses due to any cause/1,000 population during a specified time period
• Live birth rate: number of live births/1,000 population or childbearing women during a specified time period
• Case fatality rate: number of deaths due to disease X/number of people with disease X during a specified time period
• Attack rate: number of new cases of disease/number population at risk at beginning of time period during specified time period

It may also be calculated by the incidence rate multiplied by duration.

Basic equation: Cumulative Incidence = # new (incident) cases/total population at risk
Applied: CI = IR_i * t_i (where IR_i is the incidence rate, t_i is the specified time period)

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