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In epidemiology, a rate ratio, sometimes called an incidence density ratio or incidence rate ratio, is a relative difference measure used to compare the incidence rates of events occurring at any given point in time.
Incidence is usually more useful than prevalence in understanding the disease etiology: for example, if the incidence rate of a disease in a population increases, then there is a risk factor that promotes the incidence.
An infection rate or incident rate is the probability or risk of an infection in a population.
The relative risk (RR) or risk ratio is the ratio of the probability of an outcome in an exposed group to the probability of an outcome in an unexposed group. Together with risk difference and odds ratio , relative risk measures the association between the exposure and the outcome.
It is calculated as = /, where is the incidence in the population, and is the incidence in the unexposed group. [ 1 ] Equivalently it can be calculated as A F p = P e ( R R − 1 ) 1 + P e ( R R − 1 ) {\displaystyle AF_{p}={\frac {P_{e}(RR-1)}{1+P_{e}(RR-1)}}} , where P e {\displaystyle P_{e}} is the exposed proportion of the population and R ...
The relationship between incidence (rate), point prevalence (ratio) and period prevalence (ratio) is easily explained via an analogy with photography. Point prevalence is akin to a flashlit photograph: what is happening at this instant frozen in time.
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It is computed as () /, where is the incidence in the exposed group, and is the incidence in the unexposed group. If the risk of an adverse event is increased by the exposure rather than decreased, the term relative risk increase (RRI) is used, and it is computed as ( I e − I u ) / I u {\displaystyle (I_{e}-I_{u})/I_{u}} .