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Incidence Meaning

Let's discuss the definition of incidence. In epidemiology, the occurrence of new cases of disease, injury, or other medical conditions over a specified time period, usually expressed as a rate or proportion. A person developing diabetes, becoming infected with HIV, starting to smoke, or being admitted to the hospital are all examples of incident cases or events.


What is Epidemiology?

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  • It is the study of how and why diseases arise in various groups of people, and it is used to prepare and assess disease prevention measures as well as to direct the treatment of patients who have already acquired disease.

  • Epidemiology is sometimes referred to as the foundational science of public health.

  • The analysis of the distribution and determinants of health-related states or events in particular populations, as well as the application of this information to disease prevention, is known as epidemiology.

Incidence, Risk and Rate

The incidence rate is a calculation of how often a disease or other occurrence occurs within a given period of time. It's also known as the incidence density rate or person-time incidence rate since the denominator is the combined person-time of the population at risk (that is the sum of the time duration of exposure across all the number of people exposed).

There are two main measures of incidence:

Risk (or cumulative incidence)

The population at risk at the start of the study period is used to calculate risk.


The rate is estimated in person-time units and is linked to a more accurate measure of the population at risk during the study period.


What is Risk?

The proportion of people in a population (who are initially disease-free) who contract the disease within a given time period is known as risk. The probability of an incident is expressed as a percentage (or if small as per 1000 persons).


Incidence Risk = Number of new cases of disease in a given time period/ Number of disease-free people at the start of that time period


The incidence risk assumes that the entire population at risk at the start of the study period has been followed for the production of the outcome under investigation for the prescribed time period. Participants in a cohort study, on the other hand, could drop out during follow-up.

For example, some participants may:

  1. Develop the outcome under investigation

  2. Refuse to continue to participate in the study

  3. Migrate

  4. Die

  5. Enter the study some time after it starts

A more accurate metric, the incidence rate, can be estimated to account for these differences during follow-up.


Incidence Versus Prevalence

In comparison to incidence, frequency encompasses both new and current cases. An event case is someone who is recently diagnosed with diabetes, while a prevalent case is someone who has had diabetes for ten years. An individual with chronic diseases, such as diabetes, can only have one occurrence in their lifetime. An individual may have several incidences of diseases that can be cured (e.g., the common cold) over the course of his or her lifetime.


Incidence Rate = Number of new cases of disease in a given time period / Total person-time at risk during the follow-up time

Prevalence Rate = All new and pre-existing cases of a specific disease during a given time period / Total population during the same time period


The study of incident cases may reveal details about a disease's aetiology (or cause) and outcome. It also enables researchers to identify the factors that increase the risk of a disease or other medical condition. The study of prevalent cases, on the other hand, incorporates both new and surviving cases, making it difficult to determine if risk factors are the cause of new cases or the cause of survival.


The Relationship Between Prevalence and Incidence

The proportion of the population with a disease at a given point in time (prevalence) and the rate of new disease occurrence over a given period of time (incidence) are closely related.

Prevalence depends on:

  1. The incidence rate (R)

  2. The duration of disease (T)


What is Incidence Rate?

  • The incidence rate is a calculation of how often an occurrence, such as a disease or an accident, happens within a given period of time.

  • The number of new cases of a disease over a given time span, expressed as a percentage of the number of people at risk for the disease, is known as the incidence rate.

Incidence rates and prevalence proportions are commonly used to express the health status of a population. Because there are several methods for calculating these epidemiological measures, it is difficult to compare. This research looks at the effect of various numerator and denominator organisational concepts on occurrence rates and prevalence proportions.


Incidence Rate Formula

Incidence Rate = Number of new cases of disease in a given time period/ Total person-time at risk during the follow-up time


Real World Example of Incidence Rate

In 2013, a county in the United States with a population of 500,000 people may have had 20 new cases of tuberculosis (TB), resulting in an incidence rate of four cases per 100,000 people. This is higher than the overall TB incidence rate in the United States, which was 9,852 new TB cases in 2013, or three cases per 100,000 people.


Key Takeaways

  • The incidence rate is a measure of how likely an occurrence is to occur within a given time period.

  • This rate allows for the prediction of potential events and the development of contingency plans.

  • The term "incidence" differs from "prevalence," which refers to the overall number of cases rather than the likelihood of another case occurring.

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FAQs on Incidence

Question 1. What is the Meaning of Incidence in Epidemiology?

Answer: Let’s know what is incidence. Incidence study can be described as a disease measure that helps us to assess a person's likelihood of being diagnosed with a disease over a given period of time. As a result, incidence is defined as the number of newly diagnosed cases of a disease.

Question 2. What is Incidence Rate? What is Incidence Rate Formula?

Answer: Incidence rate is equal to no. of new cases/population at risk * population size. Let's go over an example: During one year, 12 men out of a population of 50,000 healthy men were newly diagnosed with colorectal cancer. Thus, our population size is 50,000. Then, the colon cancer incidence rate is equal to 24 per 100,000 men per year.

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