# Cohort Study: Incidence Rate

## Incidence Rate

(See Aschengrau and Seage, pages 44-47)

The most precise estimate of the impact of exposure in a population is called the
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incidence rate
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.

The incidence rate is defined as the number of new cases of disease during the given period of time (usually one year) over the total person-time of observation over the same period.

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# New Cases of Disease
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Total Person-Time of Observation

The denominator presents the sum of each individual's time at risk or the sum of the time that each person remained under observation and free from the disease of interest. This allows the researcher to account for those who dropped out of the study and no longer contribute to person-years at risk due to a variety of reasons (moved away, refused to participate, died from unrelated causes, etc.). At the end of follow-up period, all person-years are summed up to represent the cumulative time at risk for disease. The time at risk for each person will be calculated from the time the individual entered the study until the time he/she exits the study.

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Definition
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Scenario 1
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Scenario 2
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Scenario 3
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