[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Measures of frequency and association](https://durgical.com/en/epidemiology/areas/measures/) › Incidence

# Incidence

- Measure

How many new cases appear in a group that was disease-free at the start, over a defined time.

Updated Nov 5, 2026

## Key points

- Cumulative incidence = new cases / population at risk at the start, over a defined period.
- The population at risk excludes people who already have the disease (prevalent cases) or cannot get it.
- It always carries a time period: 5% over 2 years makes sense; 5% on its own does not.
- The incidence rate uses person-time in the denominator, for example per 1,000 person-years.
- Incidence is for studying causes; prevalence measures disease burden.

On this page

- The basics Key points
- What it is
- Formula and reading
- Worked example
- Cumulative incidence versus incidence rate
- Markers
- How to read it
- Common pitfalls
- Memory aid

- Study Test yourself

- Connections Connections

## What it is

Incidence measures new cases in a population at risk. It comes in two forms: cumulative incidence (risk), the proportion of disease-free people who become ill over a period, and the incidence rate, which divides new cases by the person-time observed. It is the measure used to study causes, because it captures who moves from healthy to ill.

## Formula and reading

- Cumulative incidence (risk) = new cases in the period / people at risk at the start. It ranges from 0 to 1 and has no units.
- Incidence rate = new cases / total person-time at risk. Its units are cases per person-time.
- The attack rate is a cumulative incidence for an outbreak, over a short period.
- Reading: a higher value means more healthy people become ill over that time.

## Worked example

- Data: a town has 2,000 adults; 200 already have hypertension at the start. Over 2 years, 90 new cases appear.
- Population at risk: 2,000 − 200 = 1,800 (people who already have hypertension cannot be new cases).
- Cumulative incidence = 90 / 1,800 = 0.05.
- Expressed as 5% over 2 years, or 50 new cases per 1,000 adults at risk over 2 years.
- Common mistake: 90 / 2,000 = 4.5% underestimates the risk because existing cases sit in the denominator.
- Reading: 1 in 20 adults without hypertension develops it over 2 years in this town.

## Cumulative incidence versus incidence rate

| Feature | Cumulative incidence | Incidence rate |
| --- | --- | --- |
| Denominator | People at risk at the start | Person-time at risk |
| Units | None (a proportion over a period) | Cases per person-time |
| Follow-up | Assumes everyone is followed for the whole period | Allows follow-up of different lengths |
| Typical use | Closed cohorts, outbreaks, trials | Open cohorts, population registries |

## Markers

- CI
- IR
- per 1,000

## How to read it

- Comparing incidence in the exposed and the unexposed gives the relative risk.
- In public health surveillance, weekly incidence shows whether an outbreak is growing or fading.

## Common pitfalls

- Leaving prevalent cases in the denominator.
- Reporting incidence without its time period.
- Calling a proportion without person-time a rate.

## Memory aid

Incidence is the front door: it counts only the people who walk from healthy to ill during the time you watch.

## Test yourself

Think of the answer, then tap the question to see it.

## Why are prevalent cases excluded from the denominator of incidence?

They already have the disease and cannot become new cases; including them underestimates the risk.

## What is missing from the statement that incidence was 3%?

The time period: cumulative incidence only makes sense over a stated time, for example 3% over one year.

## Which frequency measure is preferred for studying the causes of a disease?

Incidence, because it records the move from healthy to ill after exposure.

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## Connections

## Related 5

- [Prevalence](https://durgical.com/en/epidemiology/prevalence/)
- [Incidence rate and person-time](https://durgical.com/en/epidemiology/incidence-rate-and-person-time/)
- [Prevalence, incidence and duration](https://durgical.com/en/epidemiology/prevalence-incidence-and-duration/)
- [Relative risk](https://durgical.com/en/epidemiology/relative-risk/)
- [Attack rate](https://durgical.com/en/epidemiology/attack-rate/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Principles of Epidemiology | Lesson 3 - Section 2](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html) (Centers for Disease Control and Prevention, 2012); [Principles of Epidemiology | Lesson 3 - Section 1](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section1.html) (Centers for Disease Control and Prevention, 2012).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/epidemiology/incidence/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/incidence/