Introduction (2023)

Welcome to our comprehensive guide on epidemiological measurements to describe the occurrence of disease. In this article, we will explore the commonly used measures of disease frequency, including prevalence and incidence, as well as measures of effect. Our goal is to provide you with a detailed understanding of these measurements and their significance in epidemiology.

Measures of Disease Frequency


Prevalence is a measure that quantifies the proportion of individuals in a defined population who have a specific disease or health outcome at a given point in time or during a specified period. It helps us understand the burden of disease in a population at a particular moment.

For example, if we have 10,000 female residents in town A on January 1, 2006, and 1,000 of them have hypertension, the prevalence of hypertension among women in town A on that date would be calculated as 1,000/10,000 = 0.1 or 10%.

Prevalence is particularly useful for comparing the occurrence of diseases across different geographical areas or sub-groups of the population. However, it may not be the most suitable measure for establishing the determinants of disease in a population.


In contrast to prevalence, incidence measures the number of new cases of a disease or health outcome that develop in a population at risk during a specified time period. It provides insights into the rate at which new cases occur.

There are two main measures of incidence:

  1. Risk (or cumulative incidence): This measure is related to the population at risk at the beginning of the study period. It calculates the proportion of individuals who develop the disease within a specified time interval. Incidence risk is expressed as a percentage or per 1,000 persons.

  2. Rate: This measure provides a more precise estimation of the population at risk during the study period. It takes into account the sum of the time that each person remained under observation and at risk of developing the disease. Incidence rates are commonly measured in person-time units.

Calculation of Person-Time at Risk

Person-time at risk is the sum of each individual's time at risk of developing the disease. It is commonly expressed in person-years at risk. When a study subject develops the disease, dies, or leaves the study, they are no longer considered at risk and will no longer contribute person-time units at risk.

Person-time at risk is an important consideration when calculating incidence rates, as it accounts for variations in follow-up time and ensures accurate estimations of disease occurrence.

Issues in Defining the Population at Risk

For any measure of disease frequency, it is crucial to define the denominator accurately. The population at risk should include all individuals who are at risk of developing the disease under investigation. Individuals who already have the disease or are immune should be excluded from the denominator.

However, in practice, it may not always be possible to exclude individuals who are not at risk. In such cases, the resulting measure of disease frequency may underestimate the true incidence of the disease in the population under investigation.

The Relationship Between Prevalence and Incidence

Prevalence and incidence are closely related measures that provide insights into disease occurrence. The proportion of the population with a disease at a specific point in time (prevalence) and the rate of new disease occurrences during a period (incidence) are interconnected.

The relationship between prevalence and incidence depends on two factors:

  1. Incidence rate (r): The rate at which new cases of the disease occur.
  2. Duration of disease (T): The length of time from diagnosis to recovery or death.

If the incidence of a disease is low but the duration of the disease is long, the prevalence will be high relative to the incidence. On the other hand, if the incidence is high and the duration is short, the prevalence will be low relative to the incidence.

Understanding the relationship between prevalence and incidence is essential for interpreting disease occurrence and planning public health interventions.

Other Commonly Used Measures of Disease Frequency in Epidemiology

In addition to prevalence and incidence, there are other measures of disease frequency used in epidemiological studies to assess the strength of an association between a potential risk factor and the occurrence of disease. These measures are often referred to as measures of effect.

The most commonly used measure of effect is the ratio of incidence rates, which compares the rate of disease in an exposed group to the rate in an unexposed group. This measure, known as the relative risk, helps determine the likelihood of a causal association between the exposure and the disease.


In this article, we have explored the commonly used measures of disease frequency in epidemiology, including prevalence and incidence. We have also discussed the relationship between these measures and the importance of accurately defining the population at risk. Additionally, we touched upon other measures of effect used to assess the strength of associations between risk factors and disease occurrence.

By understanding these measurements, researchers and public health professionals can gain valuable insights into the occurrence and distribution of diseases, leading to more effective prevention and control strategies.

Remember, epidemiology plays a crucial role in understanding and addressing the health challenges faced by populations worldwide.

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