Incidence
- Measure
How many new cases appear in a group that was disease-free at the start, over a defined time.
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.
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.
Connections
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 (Centers for Disease Control and Prevention, 2012); Principles of Epidemiology | Lesson 3 - Section 1 (Centers for Disease Control and Prevention, 2012).
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Representative model, for educational purposes only.