Concept Architecture
Concept
Theoretically, Incidence is the occurrence of new cases of a disease or health outcome within a defined population during a specified period. It is a fundamental measure of disease occurrence in epidemiology and quantifies the transition from disease-free to diseased status. Incidence exists to measure disease risk, monitor emerging health problems and evaluate the effectiveness of preventive interventions.
Mathematically, incidence is represented using either cumulative incidence or incidence rate, depending on the study design and available follow-up data. Cumulative incidence measures the probability of developing disease over a specified period, whereas incidence rate incorporates the amount of person-time contributed by individuals at risk. Both measures quantify the frequency of new disease occurrence but differ in their treatment of follow-up time.
In practice, incidence is estimated using cohort studies, disease registries, surveillance systems and clinical trials by identifying new cases arising within an at-risk population. Health economists use incidence estimates to model disease burden, forecast healthcare demand, parameterise decision-analytic models and evaluate the cost-effectiveness of prevention and treatment programmes.
Purpose
Used to quantify the occurrence of new disease cases within a population, estimate disease risk and provide epidemiological inputs for health economic evaluation, decision modelling and public health planning.
Mathematical Formulae
Primary Formula
Cumulative Incidence = Number of new cases during the time period � Population at risk at the beginning of the time period
Supporting Formulae
Incidence Rate:
IR = Number of new cases � Total person-time at risk
Expressed as a percentage:
Cumulative Incidence (%) = (Number of new cases � Population at risk) ? 100
Related Mathematical Methods
Cumulative Incidence
Incidence Rate
Person-Time Analysis
Kaplan-Meier Estimator
Life Table Method
Survival Analysis
Example
A cohort of 5,000 individuals without cardiovascular disease is followed for one year. During follow-up, 125 participants experience their first myocardial infarction.
Cumulative Incidence = 125 � 5,000
Cumulative Incidence = 0.025
Cumulative Incidence = 2.5%
The one-year incidence of myocardial infarction is therefore 2.5%.
Excel Implementation
| Function | Example Formula | Health Economics Application |
|---|---|---|
| COUNTIF | =COUNTIF(B2:B5001,"Case") | Count new disease cases |
| COUNTA | =COUNTA(A2:A5001) | Count individuals initially at risk |
| IFERROR | =IFERROR(COUNTIF(B2:B5001,"Case")/COUNTA(A2:A5001),0) | Calculate cumulative incidence |
| ROUND | =ROUND(C2*100,2) | Express incidence as a percentage |
| COUNTIFS | =COUNTIFS(B2:B5001,"Case",C2:C5001,"<=365") | Calculate incidence during a defined follow-up period |
VBA (Optional)
VBA can automate incidence calculations across multiple diseases, populations and follow-up periods while generating epidemiological summary reports.
Sources
- Rothman KJ, Greenland S, Lash TL. Modern Epidemiology.
- Gordis L. Epidemiology.
- Porta M, ed. A Dictionary of Epidemiology.
- World Health Organization. Basic Epidemiology.
- Briggs A, Claxton K, Sculpher M. Decision Modelling for Health Economic Evaluation.
Related Concepts (2)
Library
Tools & Resources
1
IHME GBD Compare / Viz Hub — Institute for Health Metrics and Evaluation (IHME), Interactive visualisation ed., 2024 (University of Washington)
An interactive data-visualisation tool for exploring the Global Burden of Disease study — mortality, DALYs, risk factors and disease burden by country, age, sex and year, via treemaps, maps and trend charts — a key input to epidemiological and economic models.
Interactive DashboardView source →
Frequently Asked Questions (6)
What is incidence?
The number of new cases of a disease occurring within a defined population over a specified period, unlike prevalence, which counts existing cases.
Source: Rothman KJ, Greenland S, Lash TL. Modern Epidemiology. 3rd ed. Lippincott Williams & Wilkins; 2008.
Why does incidence count only new cases?
Incidence counts the new cases of a disease that arise in a population over a period, capturing the rate at which people are newly falling ill rather than how many are ill in total. It counts only new cases because it measures the force with which a disease is occurring, which is what reveals its causes and whether they are being controlled. A rising incidence signals a growing problem or a new risk, information that a count of existing cases would blur. It measures the flow of new disease. Rothman and colleagues (2008) describe this.
Source: Rothman et al. 2008
How is incidence measured?
Incidence is measured by counting new cases arising in a population over a period among those at risk, expressed either as cumulative incidence, the number of new cases divided by the number at risk at the start, giving a proportion or risk, or as an incidence rate, the number of new cases divided by the total person-time at risk, accounting for varying follow-up. The choice depends on whether a simple risk over a fixed period or a rate handling different follow-up times is wanted. So incidence quantifies new-case occurrence, using either a proportion or a rate.
Source: Rothman, Greenland & Lash 2008
How does incidence differ from prevalence?
Incidence measures new cases arising over a period, reflecting the risk or rate of developing the disease, while prevalence measures existing cases at a point or over a period, reflecting the burden of disease present. Incidence counts only new cases among those at risk, whereas prevalence counts all cases, new and pre-existing. Prevalence depends on both incidence and how long cases last, so a disease with high incidence but short duration may have low prevalence. So incidence concerns the occurrence of new disease, and prevalence the amount of disease present.
Source: Rothman, Greenland & Lash 2008
Why is incidence important?
Incidence is important because it measures the risk and rate of developing disease, which is central to studying causes and risk factors, since analytic epidemiology compares incidence between exposed and unexposed groups. It reflects the appearance of new cases, informing the dynamics of disease and the impact of interventions that prevent new cases. Incidence also supports planning by indicating how many new cases to expect. So incidence is a fundamental measure for understanding disease risk, investigating causes, and evaluating prevention, distinct from prevalence, which measures existing burden.
Source: Rothman, Greenland & Lash 2008
What is the relationship between incidence and prevalence?
The relationship between incidence and prevalence is that prevalence depends on both the incidence of new cases and the duration of the disease: broadly, prevalence reflects incidence multiplied by average duration, so a disease with high incidence but short duration, whether through quick recovery or death, has lower prevalence than one with the same incidence but long duration. So incidence drives the appearance of cases, and their duration determines how many accumulate as existing cases. Understanding this relationship clarifies why incidence and prevalence can differ and how each reflects different aspects of disease occurrence.
Source: Rothman, Greenland & Lash 2008
Trust Record
Verified by Dr Darrin Baines
British health economist
Professional identity: darrinbaines.org
Verification date: 21 Nov 2025
Content version: 1.0.0
Canonical Identity
- Persistent URI
- https://healtheconomics.wiki/concept/incidence
- Term code
- HE-ES-EP-019
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