What is the difference between incidence rate and cumulative incidence?

Cumulative incidence (incidence proportion, risk or attack rate) divides new cases by the population at risk at the start of the period. The incidence rate (incidence density) divides new cases by the total person-time at risk — for example patient days — so it accounts for people being observed for different lengths of time.

What is incidence density in infection prevention?

Incidence density is another name for the incidence rate. Healthcare-associated infection rates such as infections per 1,000 patient days or per 1,000 device days are incidence densities.

What is the difference between incidence and prevalence?

Incidence counts new cases over a period; prevalence counts all existing cases — new and pre-existing — at a point or during a period.

Free tool · CIC® exam topic

Incidence rate calculator

New cases divided by person-time at risk — or by the population at risk for cumulative incidence — scaled to the unit you report.

Incidence rate (person-time)

Result

Incidence rate: 1.43 per 1,000

6 new cases over 4,200 units of person-time.

Cumulative incidence (risk)

Result

Cumulative incidence: 4.00%

Incidence rate formula

CDC defines the incidence rate (person-time rate) as:

Incidence rate = new cases during the period ÷ time each person was observed, totaled for all persons

Multiply by 1,000 (or another power of 10) to report it per 1,000 person-days or person-years. In hospitals, patient days are the usual person-time; for device-associated infections it is device days.

Worked examples

CDC’s example: 16 new cases over 6,400 person-years = 2.50 per 1,000 person-years.
6 hospital-onset cases over 4,200 patient days = 1.43 per 1,000 patient days.
Cumulative incidence: 12 infections among 300 patients at risk = 4.00%.

Cumulative incidence formula

Cumulative incidence = new cases during the period ÷ population at risk at the start of the period

Also called incidence proportion, risk or — in outbreaks — attack rate. Use it when everyone is followed for the same period.

Test yourself: a CIC-style question

Incidence density (incidence rate) uses which denominator?

Number of persons at the start
Person-time at risk (e.g., patient days or device days)
Number of beds
Total admissions only

Show answer

B. Person-time at risk (e.g., patient days or device days)

Incidence density uses person-time at risk in the denominator — for example 1,000 patient days or device days — so it accounts for different lengths of observation and exposure.

Questions like this come up in the Surveillance and Epidemiologic Investigation domain of the CIC® exam. The 654 questions in the app each come with a written explanation.

FAQ

What is the difference between incidence rate and cumulative incidence?: Cumulative incidence (incidence proportion, risk or attack rate) divides new cases by the population at risk at the start of the period. The incidence rate (incidence density) divides new cases by the total person-time at risk — for example patient days — so it accounts for people being observed for different lengths of time.
What is incidence density in infection prevention?: Incidence density is another name for the incidence rate. Healthcare-associated infection rates such as infections per 1,000 patient days or per 1,000 device days are incidence densities.
What is the difference between incidence and prevalence?: Incidence counts new cases over a period; prevalence counts all existing cases — new and pre-existing — at a point or during a period.

Sources

CDC. Principles of Epidemiology in Public Health Practice (SS1978), Lesson 3: Measures of Risk

Checked against these sources on October 21, 2026. For education and CIC® exam preparation only — not a substitute for your facility’s policies, NHSN’s official reports, or clinical judgment. Calculations run in your browser; nothing you enter is sent anywhere.