3. Incidence

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2. Measurements of incidence

2.3. Incidence rate

Incidence rate or person-time rate is a measure of incidence that incorporates time directly into the denominator, i.e., a measure of the number of new cases ("incidence") per unit of time ("rate").

Similar to the incidence proportion the incidence rate numerator is the number of new cases identified during the period of observation. The denominator, however, is the total person-time, or the amount of time that all at-risk persons were in the study. It is the sum of the time each person was observed, totalled for all persons and therefore actually a combination of the number of people and the amount of time. Thus, the incidence rate is the ratio of the number of new cases to the total time the population is at risk of disease. An example for how this can be calculated is given in the next section.