In epidemiology, the number of cases of disease, infection, or some other event having their onset during a prescribed period of time in relation to the unit of population in which they occur. Incidence measures morbidity or other events as they happen over a period of time. Examples include the number of accidents occurring in a manufacturing plant during a year in relation to the number of employees in the plant, or the number of cases of mumps occurring in a school during a month in relation to the number of pupils enrolled in the school. It usually refers only to the number of new cases, particularly of chronic diseases. Hospitals also track certain risk management or quality problems with a system called incidence reporting.