Incidence is the number of new cases of a disease during a given time interval, usually one year. It can be expressed as a proportion or as a rate.Incidence proportion (also known as risk) is the number of new cases divided by the size of the population at risk. For example, if a stable population contains 1,000 persons and 28 develop a condition over two years of observation, the incidence proportion is 28 cases per 1,000 persons.The incidence rate is the number of new cases per unit of person-time at risk. In the same example as above, the incidence rate is 14 cases per 1000 person-years, because the incidence proportion (28 per 1,000) is divided by the number of years (two). Using person-time rather than just time handles situations where some people drop out of a study.
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