Incidence Rate
Definition: Incidence rate is the pace or intensity of accumulation of disease cases. In other words, incidence rate measures how fast a disease is spreading.
Incidence rate is calculated using the following formula:
(Number of New Cases) / (Person-Time at Risk)
- The numerator (Number of New Cases) is a straightforward count of new cases.
- The denominator (Person-Time at Risk) is measured as follows.
Person-Time: The amount of at risk time each person contributes
- 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 as best and often impossible. So 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 or 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.
