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.
References
Centers for Disease Control and Prevention. Principles of Epidemiology in Public Health Practice. 3rd ed.
Gordis L. Epidemiology. 6th ed. Elsevier; 2019.