Incidence (epidemiology)

The incidence of a medical condition refers to the number of new cases in a specified population over a specific period of time. It is a fundamental concept in epidemiology and is distinguished from prevalence, which refers to all cases, new and existing, in the population at a given time.

Relative incidence of cutaneous cysts

Definition

In epidemiology, incidence is a measure of the probability of occurrence of a given medical condition in a population within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate.

Calculation

The calculation of incidence involves:

• The number of new cases that occur in a defined period
• The size of the population initially at risk.

Types of Incidence

There are two types of incidence rates:

1. Cumulative incidence, also called incidence proportion, is the number of new cases within a specified time period divided by the number of people at risk in the population at the beginning of the study.
2. Incidence rate is the number of new cases within a specified time period divided by the time at risk in the population at risk.

Incidence vs Prevalence

Incidence should not be confused with prevalence, which is a measure of the total number of cases of disease in a population at a given time rather than new cases.

See Also

• Epidemiology
• Prevalence
• Morbidity rate
• Mortality rate