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| Block 1: Epidemiology |
| EPI3-6: FOUR MEASURES OF EFFECT: THE INCIDENCE RATE RATIO |
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| Block 1: Epidemiology |
| EPI3-6: FOUR MEASURES OF EFFECT: THE INCIDENCE RATE RATIO |
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The incidence rate, IR, (an alternative term is incidence density), is defined as:
The sum of the time periods in the denominator is often measured in years and is referred to as person-years, person-time, or risk time. For each individual in the population the time at risk is the time during which that individual is in the study population and remains free from the disease, and therefore at risk to get it. These time periods at risk are then summed for all individuals.
As before, we can define the Incidence Rate Ratio:
1813 workers from a cohort followed up for 10 years over 90 635 person years in a high noise exposure zone developed hearing loss in the 10 year follow up period. 952 workers from a cohort followed up for 10 years with 95 163 person years of observation in a lower noise exposure zone developed hearing loss in the same period. The respective incidence rates are 0.02 and 0.01 for high and low exposure cohorts respectively.
The incidence rate ratio is 2.
