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Person-Time Units

  • In: Encyclopedia of Epidemiology
  • Edited by: Sarah Boslaugh
  • Subject:Public Health (general), Public Health Research Methods , Epidemiology & Biostatistics

It is common in medicine and epidemiology to express the frequency of occurrence of some event in terms of the number of events per person-time unit; for instance, the number of complications per 100 patient-days or the number of deaths per 100,000 person-years. A good example is the incidence rate, also known as the incidence density or force of morbidity or mortality. The incidence rate is calculated as

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The numerator is always the number of new cases of the disease in the time period studied, and the denominator is the sum of the time of observation for all the subjects in the study. This fraction is usually converted to a standard unit such as cases per 100 to facilitate comparisons.

Person-time units are used when the ...

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