Cumulative Incidence/ Cumulative Risk/ Risk
Definition: Cumulative incidence is a measure of disease frequency that addresses the question "How far has the disease spread during a specified period of time?" It is calculated using the following formula:
(Number of new cases) / (Total population at risk)
Example: There is a population at risk composed of 100 senators that we followed for one year. Twenty-five senators developed symptoms consistent with inhalation anthrax disease. Laboratory testing confirmed that they were infected with Bacillus anthracis.
The one-year 'risk' (cumulative incidence) of anthrax among senators is the number of senators who were diagnosed with anthrax divided by the number of senators at risk at the beginning of the follow-up period.
- Since there were no existing cases of anthrax at the beginning of the year all 25 cases are considered to be "new" cases (the numerator).
- The population at risk, in this case, is 100 senators and all were at risk of anthrax at the beginning of the time period.
- The calculation for cumulative incidence of anthrax in this situation is: 25 cases / 100 senators at risk = 25%.
Interpretations
- In the year in question, anthrax spread to 25% of the Senators.
- In the year in question, senators had a 25% risk of developing anthrax.
