Incidence concerns new cases and the population initially at risk. Prevalence concerns all existing cases in the population at a defined time or during a period.
Side-by-side comparison
| Feature | Incidence | Prevalence |
|---|---|---|
| Main question | How quickly are new cases occurring? | How common is the condition? |
| Numerator | New cases | All existing cases |
| Denominator | Population at risk or person-time | Total defined population |
| Useful for | Etiology, prevention, surveillance | Service planning and burden |
Incidence measures
Cumulative incidence is the proportion of an initially disease-free population that develops the outcome during a stated period.
Risk = New cases / Population at risk at the startIncidence rate uses person-time and accommodates different follow-up lengths.
Incidence rate = New cases / Total person-time at riskPrevalence measures
Point prevalence describes cases at a particular moment. Period prevalence includes anyone who had the condition during a stated period.
Prevalence = Existing cases / Total populationAlways state the reference date or period and define whether the denominator represents residents, registered patients, attendees, workers, or another population.
Worked example
At the start of a year, 1,000 disease-free workers are followed. Fifty develop dermatitis. At year end, 80 workers have dermatitis, including cases that began earlier.
- Cumulative incidence: 50 / 1,000 = 5% during the year.
- Year-end prevalence: 80 / the year-end workforce denominator.
The figures differ because incidence counts onset, while prevalence reflects both onset and duration.
How duration affects prevalence
In a stable population with a relatively uncommon condition, prevalence is approximately incidence multiplied by average duration. Long-lasting diseases may have high prevalence even when few new cases occur. Rapid recovery or death can keep prevalence low despite high incidence.
Common mistakes
- Calling a cross-sectional proportion an incidence rate.
- Including people who cannot develop the outcome in the at-risk denominator.
- Reporting a rate without its time unit or multiplier.
- Comparing populations with different case definitions or ascertainment.
Use the calculators
If your data include new cases and person-time at risk, use the Incidence Rate Calculator. If your data are existing cases divided by a defined population at a point or period, use the Prevalence Calculator.
For research planning, the Study Design Selector can help decide whether your question is about disease occurrence over time, existing burden, association, or program performance.
References
Centers for Disease Control and Prevention. Principles of Epidemiology in Public Health Practice. 3rd ed.
Gordis L. Epidemiology. 6th ed. Elsevier; 2019.