Epidemiology, Prevalence and Incidence
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Epidemiology
It is the study of how often diseases occur in different groups of people and why, and is used to plan and evaluate strategies to prevent illness and as a guide to the management of patients in whom disease has already developed.[1] Epidemiology is often described as the basic science of public health.[2]
Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems[3]
The CDC[2] break this definition down into the important principles of epidemiology:
Study – a scientific discipline with sound methods of scientific inquiry at its foundation. Various methods can be used to carry out epidemiological investigations: surveillance and descriptive studies can be used to study distribution; analytical studies are used to study determinants.[4]
Distribution – the frequency (number of health events in a population and it’s relationship of that number to the size of the population – prevalence) and pattern (the occurrence of health-related events by time, place, and person – incidence) of health events in a population.
Determinants – the causes and other factors that influence the occurrence of disease and other health-related events.
Health-related states or events – these may include communicable and non-communicable diseases, chronic diseases, injuries, birth defects, maternal-child health, occupational health, and environmental health. More recently studies have included behaviours related to health and well-being and genetic markers of disease risk.
Specified populations – the epidemiologist is concerned about the collective health of the people in a community or population[2]. A key feature of epidemiology is the measurement of disease outcomes in relation to a population at risk.[1] Implicit in any epidemiological investigation is the notion of a target population about which conclusions are to be drawn and are often observations that can only be made on a study sample, which is selected in some way from the target population.[1]
Application – Epidemiology is not just “the study of” health in a population; it also involves applying the knowledge gained by the studies to community-based practice.
Epidemiology is the study (scientific, systematic, data-driven) of the distribution (frequency, pattern) and determinants (causes, risk factors) of health-related states and events (not just diseases) in specified populations (patient is community, individuals viewed collectively), and the application of (since epidemiology is a discipline within public health) this study to the control of health problems.[2]
Prevalence
Prevalence measures how much of a disease or condition there is in a population at a particular point in time.
Prevalence gives a figure for a factor (disease, injury, health status etc) at a single point in time (point prevalence) or time period (period prevalence). Period prevalence provides the better measure of the factor since it includes all cases between two dates, whereas point prevalence only counts cases on a particular date. It is a measure of disease that allows us to determine a person’s likelihood of having a disease. It is most meaningfully reported as the number of cases as a fraction of the total population at risk and can be further categorised according to different subsets of the population.
- An example of prevalence: A recent Scottish study showed that the prevalence of obesity in a group of children aged from 3 to 4 years was 12.8% at the time.[5]
Incidence
Incidence measures the rate of occurrence of new cases of a disease or condition.
Incidence is the number of instances of a factor (disease, injury, health status etc) during a given period (day, month, year, decade) in a specified population (age group, community, country etc). Incidence can tell us how many cases of a particular factor have been suffered by a specified population in a given period of time.[6] It is a measure of disease that allows us to determine a person’s probability of being diagnosed with a disease during a given period of time or it might tell us how patterns of a condition within a population change over time.[6] Incidence is usually expressed as a rate, something that is measured within a set number of people and in a time period.
- An example of incidence: Auckland in New Zealand, often has epidemics of meningococcal disease, with annual incidences of up to 16.9/ 100,000 people.[7]
Incidence versus Prevalence
Incidence is often confused with prevalence. The easy way to remember the difference is that prevalence is the proportion of cases in the population at a given time rather than 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.