Incidence rate

Incidence rate is a measure used in epidemiology to determine the frequency at which new events occur in a population. It is often used in the context of disease, to measure the probability of a disease occurring in a certain population over a specific period of time.

Definition

The incidence rate is defined as the number of new cases of a disease or condition in a population during a specific time period, divided by the number of individuals at risk of developing the disease or condition during that same time period. The time period is often one year, but can be any length of time. The incidence rate is usually expressed per 1,000 or 100,000 individuals at risk.

Calculation

To calculate the incidence rate, the number of new cases during a specified time period is divided by the number of individuals at risk during that time period. The result is then multiplied by a standard size of population (usually 1,000 or 100,000) to make the rate easier to compare with other rates.

Uses

Incidence rates are used in epidemiology to:

• Measure the risk of individuals in a population developing a disease or condition.
• Compare the disease risk in different populations or groups.
• Monitor changes in disease risk over time.
• Identify causes and risk factors for disease.

Limitations

While incidence rates are a useful measure in epidemiology, they have some limitations. They can be influenced by changes in population size and age distribution, and by changes in diagnostic criteria or reporting practices. They also do not provide information on the duration or severity of disease, or on the number of individuals who have the disease at any one time.

See also

• Prevalence
• Epidemiology
• Morbidity rate
• Mortality rate