Evidence-appraisal glossary
Incidence rate
An incidence rate is the number of new cases of an outcome divided by the total person-time at risk. Unlike a proportion, it has units of cases per unit of time (for example, per 1000 person-years) and is not bounded between 0 and 1.
Also called: incidence density.
The incidence rate, sometimes called incidence density, measures how quickly new cases arise in a population, accounting for the fact that people are observed for different durations. It differs from cumulative incidence, which is a probability between 0 and 1 that answers what fraction of a fixed group develops the outcome over a set period. A frequent error is reading a rate as a risk: a rate of 200 cases per 1000 person-years does not mean 20 percent of people get the disease, because the same rate can come from many people followed briefly or few people followed a long time.
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This is a plain-language methodology definition for reading research. It is general education, not medical advice.