In epidemiology, health research, and statistical analysis, incidence refers to the occurrence of new cases in a population over a specified time. Analysts report it either as a raw count, a proportion (cumulative incidence), or a rate (incidence rate).
While prevalence captures all existing cases, incidence focuses strictly on newly occurring cases. It helps researchers and public health professionals understand how rapidly a condition is spreading, whether interventions are working, and how risks differ across groups.
Key Characteristics of Incidence
- It is typically calculated as:
Incidence = (Number of new cases during a time period) ÷ (Number of people at risk during that period)
- Requires a clearly defined time frame (e.g., new cases per year).
- Includes only people who were initially at risk (i.e., did not already have the condition).
- Reflects rate of occurrence, not an overall total.
There are two main types:
- Cumulative incidence: The proportion of people who develop the condition over a set time frame.
- Incidence rate (or density): Accounts for person-time and allows for varying follow-up periods among individuals.
It is especially useful for identifying emerging health threats, comparing risk across populations, and evaluating the impact of preventive strategies. Unlike prevalence, it is not affected by how long the condition lasts—only how often it appears.
Example
For example, in a year-long study of 5,000 people who did not have diabetes at the start, 200 are newly diagnosed during the year. The incidence is:
200 ÷ 5,000 = 0.04, or 4% per year
This means that 4% of the at-risk population developed diabetes during the one-year study period.
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