# Incidence (epidemiology)

Editor-In-Chief: C. Michael Gibson, M.S., M.D. ; Assistant Editor(s)-In-Chief: Kristin Feeney, B.S.

## Overview

Incidence is a measure of disease frequency that counts the occurrence of new cases of disease that develop in a candidate population over a specified time period. It can be generally referred to as the risk of developing a new case. Although sometimes loosely expressed simply as the number of new cases during a specified time period, it is better expressed as a proportion or rate in which the calculation is a division of two related numbers with the numerator being a subset of the denominator.

## Calculating Incidence

Incidence has three key components:

1. Incidence measures new disease events. In the case that a disease occurs more than once, incidence often measures the first occurrence of that disease.
2. New cases are measured in a candidate population which can be defined as a population of people who are at-risk for getting the disease. Risk is often defined by the an investigator's interest and relates to the
3. Incidence accounts for a specific amount of time that the members of the at-risk population are followed. Incidence measures an individual's transition from healthy to disease state. It is, therefore, important to have a metric to quantify the time that must pass for this transition to occur.

## Types of Incidence Measures

• Cumulative incidence is the proportion of a candidate population that becomes disease over a specific period of time. Cumulative incidence is often thought of as the average risk of getting a disease over a certain time period, whereby risk is defined as the probability of getting the disease. As a function of time, cumulative incidence is influenced by the length of follow-up of the candidate population. For example, if a population initially contains 1,000 non-diseased persons and 28 develop a condition over two years of observation, the incidence proportion is 28 cases per 1,000 persons, i.e. 2.8%.
• Incidence rate is the occurrence of new cases of disease that arise during a person-time of observation/period of time at-risk. Person-time can be understood as the time acrueed only among candidates for the disease. Therefore, person-time attributes time to the denominator of an incidence rate only up until the individual becomes diagnosed with the disease of interest. In the same example as above, the incidence rate is 14 cases per 1000 person-years, because the incidence proportion (28 per 1,000) is divided by the number of years (two). 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. Use of this measure implicitly implies the assumption that the incidence rate is constant over different periods of time, such that for an incidence rate of 14 per 1000 persons-years, 14 cases would be expected for 1000 persons observed for 1 year or 50 persons observed for 20 years.

## Critical Considerations

• When this incidence rate assumption is substantially violated, such as in describing survival after diagnosis of metastatic cancer, it may be more useful to present incidence data in a plot of cumulative incidence over time, taking into account loss to follow-up, using a Kaplan-Meier Plot.
• Incidence should not be confused with prevalence, which is a measure of the total number of cases of disease in a population, rather than the rate of occurrence of new cases. Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence indicates how widespread the disease is.
For example, consider a disease that takes a long time to cure, and that was spread widely in 2002, but whose spread was arrested in 2003. This disease will have a high prevalence and a high incidence in 2002; but in 2003 it will have a low incidence, although it will continue to have a high prevalence because it takes a long time to cure. In contrast, a disease that has a short duration may have a low prevalence and a high incidence.
• When studying etiology of a disease, it is better to analyze incidence rather than prevalence, since prevalence mixes in the duration of a condition, rather than providing a pure measure of risk.

### Distinguishing Characteristics of Incidence and Prevalence

 Measure Type of Number Units Range Type of Cases Major Applications Cumulative Incidence (CI) Proportion None 0-1 New Research studies involving causation, prevention, and treatment of a disease Incidence Rate (IR) True Rate 1/time or t-1 0 to ∞ New Research studies involving causation, prevention, and treatment of a disease Prevalence Proportion None 0 to 1 Existing Resource management/planning or general ecological evaluation of a disease