Incidence Rate Definition

The incidence rate is a measure of the frequency with which a disease or other incident occurs over a specified time period. Incidence rate or “incidence” is numerically defined as the number of new cases of a disease within a time period, as a proportion of the number of people at risk for the disease.

When the denominator is the sum of the person-time of the at-risk population, it is also known as the incidence density rate or person-time incidence rate. Using person-time rather than just time handles situations where the amount of observation time differs between people, or when the population at risk varies with time.

The incidence rate is typically expressed as the number of cases per person-year of observation. Only new cases are considered when computing the incidence rate, while cases that were diagnosed earlier are excluded. The “population at risk” measure is usually obtained from census data.

Several insights can be gleaned from the incidence rate. Apart from providing information on the number of new cases of a disease or other incident that has occurred within a community, it also gives a snapshot of changes in the disease’s progression within a population over time and is thus a very important metric for tracking chronic infectious diseases such as tuberculosis and malaria. The incidence rate also enables comparisons to be made on the incidence of disease across vastly differing populations.

In the case that the incidence rate is not discussing a disease, it may cover other topics, such as foreclosures or default.

• The incidence rate measures the frequency with which an incident occurs over a specified time period.
• It is generally referred to when discussing diseases and typically expressed as the number of cases per person-year of observation.
• It offers insight into new cases of a disease within a community, but also gives a snapshot of changes in a disease’s progression over time.
• It is different from prevalence, which measures the total accumulation of cases and determines whether the disease is widespread.

For example, a county in the U.S. with a population of 500,000 may have had 20 new cases of tuberculosis (TB) in 2013, for an incidence rate of four cases per 100,000 persons. This is higher than the incidence rate of TB for the entire U.S., which recorded 9,852 new TB cases in 2013, for an incidence rate of three cases per 100,000 persons.

As an example of determining trends using incidence rates, consider a study on lung-cancer rates released in January 2014 by the Centers for Disease Control and Prevention (CDC). The study found that thanks to tobacco control efforts, lung cancer incidence rates from 2005 to 2009 declined by 2.6 per year among men, from 87 to 78 cases per 100,000 men; the lung cancer incidence rate fell 1.1 per year among women, from 57 to 54 cases per 100,000 women.

Incidence should not be confused with “prevalence,” which measures the number of cases of a condition or illness in a population at a certain point in time. Incidence measures something during a specific time period. Prevalence is the total accumulation of incidences over a period of time.

As an example, the incidence of loan foreclosures would be the number of foreclosed loans over a time period. Prevalence would be the total number, thus, all the incidences added up. While incidence enables an assessment to be made of the risk of contracting a disease, prevalence shows whether the disease is widespread or not.