9.4.9 Prevalence, Cumulative Incidence and Incidence Rates

Describes What is Meant by: Prevalence, Cumulative Incidence and Incidence Rates

Prevalence, cumulative incidence, and incidence rates are commonly used epidemiological measures to describe the frequency of a disease in a population.

Prevalence:

Prevalence refers to the proportion of individuals in a population who have a specific disease or health condition at a given point in time or during a specific period. It is expressed as a percentage or ratio and provides an estimate of the disease burden within the population. Prevalence takes into account both new and existing cases of the disease and is useful for understanding the overall health status of a population, allocating healthcare resources, and designing public health interventions.

Cumulative incidence:

Cumulative incidence, also known as the risk or incidence proportion, measures the probability of an individual in a population developing a specific disease or health condition during a specified time period. It represents the proportion of new cases of the disease among the disease-free population at the beginning of the time period. Cumulative incidence provides information about the risk of developing the disease and can be used to identify populations at risk, prioritize preventive measures, and evaluate the effectiveness of interventions. Cumulative incidence can be calculated using the following formula:

Incidence rates:

Incidence rates, or incidence density, measure the frequency at which new cases of a disease or health condition occur in a population during a specified time period. Unlike cumulative incidence, incidence rates take into account the changing size of the population at risk over time and the varying time periods in which individuals are at risk. Incidence rates are typically expressed per 1,000, 10,000, or 100,000 persons and are useful for comparing disease occurrence across different populations or time periods. Incidence rates can be calculated using the following formula:

Concept	Definition
Prevalence	The proportion of individuals in a population with a specific health condition or disease at a specific point in time. It is calculated as the number of existing cases of the condition divided by the total population at risk.
Cumulative incidence	The proportion of individuals in a population who develop a specific health condition or disease over a specified period of time. It is calculated as the number of new cases of the condition divided by the total population at risk at the beginning of the time period.
Incidence rate	The frequency with which new cases of a specific health condition or disease occur in a population over a specified period of time. It is calculated as the number of new cases of the condition divided by the total person-time at risk during the time period.

Understanding these measures is essential in epidemiological research, as they allow for comparisons between different populations and can help to identify risk factors for specific diseases.

References:

1. Porta, M. (2014). A dictionary of epidemiology. Oxford University Press.
2. Altman, D. G., & Bland, J. M. (1994). Statistics notes: The odds ratio. BMJ, 309(6966), 696-696. doi: 10.1136/bmj.309.6966.696