Epidemiology · Reference
What are incidence and prevalence?
Incidence and prevalence are the two fundamental measures of disease frequency. Incidence counts new cases arising in a population over a period; prevalence counts all existing cases at a point or over a period. The distinction between new and existing cases is central to epidemiology.
New cases versus existing cases
The core distinction is simple but crucial: incidence counts only new cases that develop during a time period, whereas prevalence counts all existing cases — new and old — present at a point in time (point prevalence) or during an interval (period prevalence). Incidence answers "how quickly are people developing this condition?"; prevalence answers "how much of this condition is present right now?". Confusing the two is a common and consequential error, because they behave very differently and are suited to different purposes.
How each is calculated
Incidence is usually expressed in one of two ways. Cumulative incidence (incidence proportion or risk) is the number of new cases divided by the number of people at risk at the start of the period. Incidence rate (incidence density) is the number of new cases divided by the total person-time at risk, which accounts for people being followed for different lengths of time.
Prevalence is a proportion: the number of existing cases divided by the total population at that time. Because it has no time dimension in the numerator, prevalence is a snapshot, not a rate.
The relationship between them
Incidence and prevalence are linked through the duration of disease. Under steady-state conditions, prevalence (P) is approximately the product of incidence (I) and the average duration of the disease (D): P ≈ I × D (more precisely, the prevalence odds equal I × D). This explains why a condition with low incidence but long duration can have high prevalence, while a short-lived condition can have high incidence yet low prevalence at any moment. A change in prevalence may therefore reflect a change in new cases, in how long cases persist, or both.
When each measure is used
Incidence is the measure of choice for studying the causes of disease and the effect of exposures, because it captures the rate of new onset that those factors might influence; it underpins risk measures such as relative risk. Prevalence is well suited to describing the burden of disease and planning services, since it reflects how many people currently have the condition. Using the right measure for the question — and stating the population, time frame and case definition — is a basic standard of epidemiological reporting.
Key facts
At a glance
- Incidence: New cases arising over a period
- Prevalence: All existing cases at a point or interval
- Incidence as: Cumulative incidence (risk) or incidence rate (per person-time)
- Relationship: P ≈ I × D (prevalence ≈ incidence × duration)
- Used for: Incidence → causes/risk; prevalence → burden/planning
Common questions
FAQ
What is the difference between incidence and prevalence?+
Incidence counts only new cases that arise in a population over a period, measuring the rate of onset, while prevalence counts all existing cases — new and pre-existing — at a given time, measuring the current burden. Incidence is about becoming a case; prevalence is about being a case.
How are incidence and prevalence related?+
They are linked by the average duration of disease. In a steady state, prevalence is approximately incidence multiplied by average duration (P ≈ I × D). A disease with low incidence but long duration can therefore have high prevalence, and a brief illness can have high incidence but low prevalence at any moment.
Which should I use to study the causes of disease?+
Incidence is generally preferred for studying causes and the effects of exposures, because it captures the rate at which new cases develop, which is what risk factors influence. Prevalence is better suited to describing disease burden and planning services.
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