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Incidence and Prevalence
 Incidence is the number of new events arising within a specified time period. The word incidence usually denotes incidence rate.
 Incidence rate is the rate at which new events occur in a population. The numerator is the number of new events arising in a specified time period; the denominator is the population at risk of an event during this time period, sometimes expressed as persontime.

 Observed annual incidence of x is therefore the number of new cases of x diagnosed in a given population over a year divided by the number of the population at risk of x during that year.
 The use of persontime as opposed to just "time" enables you to handle situations where there are dropouts in a study or where you have not been able to follow an entire cohort at risk to watch for the event under investigation. Using persontime calculations, the follow up period does not have to be the same for all individuals studied. Persontime for a group is the sum of the times of follow up for each participant in that group. See persontime incidence rate comparisons and Poisson rate confidence interval for more information.
 Prevalence is the number of events in a population at a designated time. The word prevalence usually denotes point prevalence ratio.
 Point prevalence is the number of persons with a disease or attribute at a point in time. Point prevalence ratio is expressed as below.
 Period prevalence is the number of persons who have had the disease or attribute at any time over a specified period. Period prevalence is often expressed as below but it is not strictly a ratio it is a rate.

 Prevalence ratio is equal to the incidence rate multiplied by the duration of the event.
See Last's Dictionary of Epidemiology (1995)
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