Person-Time
Definition: A measurement combining the number of persons and their time contribution in a study. This measure is most often used as denominator in incidence rates. It is the sum of individual units of time that the persons in the study population have been exposed or at risk to the conditions of interest. The most frequently used person-time is person-years.
Person time can be calculated using the information below:
- If a person develops the disease on day 2, they contribute 1.5 person-days during which they were at risk for developing disease
- If a person is at risk for 30 days and does not contract the disease, they contribute 30 person-days at risk
- Combined, these two people contributed 31.5 person days at risk
Directly calculating person-time is tedious at best and often impossible. We can estimate person-time using the following formula:
Person-time estimate
[ (Number of people at risk at the beginning of the time interval +
Number of people at risk at the end of the time interval ) / 2 ] x (Number of time units in the time interval)
Example: A population at risk is composed of 100 senators. Twenty-five senators develop symptoms consistent with inhalation anthrax disease and are confirmed by laboratory testing to have been infected with Bacillus anthracis. If 12 senators developed anthrax in September and 13 developed anthrax in October, what is the incidence rate of anthrax for those two months?
In this case,
- Numerator is the 25 new cases
- Denominator (person-time at risk) could be calculated by:
[ (100 Senators at risk at the beginning of Sept. + 75 Senators at risk at the end of Oct.) / 2 ] x 2 months = [ ( 175 / 2) x 2 ] months = 175 person-months of risk
Note: Since 25 Senators got anthrax in September and October, there are 75 Senators remaining at risk at the end of October.
The incidence rate would then be:
(25 new cases) / (175 person-months of risk) = 14% of the senators are getting anthrax each month.
