Incidence
Say it: IN-sih-duns
Incidence is the number of new cases of a disease that arise in a population at risk during a specified period of time.
Definition
What is incidence?
In epidemiology, incidence measures how often new cases of a disease or condition occur. Only people who develop the condition during the period of observation count; those who already had it at the start are left out, since they cannot become new cases. Incidence therefore reflects risk and the speed at which disease is appearing, and it is central to studying causes and outbreaks.
There are two main versions. Incidence proportion, also called cumulative incidence or risk, divides the number of new cases by the population at risk at the start of the period, for example 5 new cases per 1,000 people per year. In an outbreak it is often called the attack rate. Incidence rate divides new cases by person-time, the total time each person was followed while at risk, which copes with people joining or leaving a study at different times. The denominator should include only people who could get the disease, so ovarian cancer incidence is calculated among women.
Incidence differs from prevalence, which counts all existing cases at a given time. A disease can have low incidence but high prevalence if people live with it for many years.
Word parts
Where does the word “incidence” come from?
- in-
- on, uponLatin in
- -cid-
- to fallLatin cadere, via incidere, to fall upon
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Related
Terms that go with it
- PrevalencePrevalence is the share of a population living with a particular disease or condition, either at one point in time or across a set period.
- EpidemiologyEpidemiology is the study of how often diseases and other health events occur in populations, what causes them and how they can be controlled.
- Cohort studyA cohort study is an observational study that follows groups differing in an exposure over time to compare how often they develop an outcome.
- EpidemicAn epidemic is a rise in cases of a disease, often sudden, above the level normally expected in a given population, area and time.
- Public healthPublic health is the science and practice of preventing disease and injury and promoting health across whole communities rather than individual patients.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- CDC Principles of Epidemiology, Lesson 3 Section 2: Morbidity Frequency Measures
- NCI Dictionary of Cancer Terms: incidence
- NIMH: What is Prevalence?
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