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.
Incidence is sometimes used alone as a shorthand for incidence rate. Although this is sloppy usage, it is frequently encountered.
Incidence should not be confused with prevalence, which is a measure of the total number of cases of disease in a population, rather than the rate of occurrence of new cases. Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence tells us how widespread the disease is.
For example, consider a disease that takes a long time to cure, and that was spread widely in 2002, but whose spread was arrested in 2003. This disease will have a high prevalence and a high incidence in 2002; but in 2003 it will have a low incidence, although it will continue to have a high prevalence because it takes a long time to cure. In contrast, a disease that has a short duration may have a low prevalence and a high incidence.
Generally speaking, diseases of short duration are better measured with incidence rates, whereas long-lasting or hereditary diseases are better measured with prevalence rates.