Epidemiology pillar · 28 definitions
Epidemiology & public health
Clear, citable definitions of the measures, study designs and concepts that epidemiology and public health are built on — from incidence and prevalence to relative risk, confounding, health equity and the social determinants of health. A research-methods reference, written to be accurate and strictly non-clinical.
Measuring health in populations
Epidemiology begins with counting. Incidence and prevalence describe how often a condition arises and how widespread it is; mortality and morbidity measure death and illness; and the case fatality rate captures severity. These measures of frequency are the foundation for comparing groups and tracking change over time.
Studying causes and associations
To investigate why health differs between groups, epidemiologists use defined study designs — chiefly the cohort study and the case-control study — and measures of association such as relative risk and the odds ratio. Sound inference depends on recognising threats such as confounding and selection bias, and on transparent reporting against guidelines such as STROBE.
Population health and equity
Beyond measurement, public health concerns the conditions that shape health across whole populations: health equity, the social determinants of health, and the levels of prevention through which interventions act. The definitions below cover both the analytic toolkit and these population-level concepts.
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Epidemiology definitions
Health equity
What is health equity?
Health equity is the principle that everyone should have a fair and just opportunity to be as healthy as possible. It means removing avoidable, unfair and remediable differences in health between groups defined socially, economically or geographically.
Read →Social determinants
What are social determinants of health?
Social determinants of health are the non-medical conditions in which people are born, grow, live, work and age. These conditions — shaped by money, power and resources — are widely recognised as major drivers of health outcomes and of avoidable differences in health between groups.
Read →Public health
What is public health?
Public health is the science and art of preventing disease, prolonging life and promoting health through the organised efforts of society. It focuses on protecting and improving the health of whole populations and communities rather than treating individual patients.
Read →Universal health coverage
What is universal health coverage?
Universal health coverage means that all people can use the health services they need — promotive, preventive, curative, rehabilitative and palliative — of sufficient quality, without suffering financial hardship. It is a population-level policy goal rather than a single scheme or programme.
Read →Herd immunity
What is herd immunity?
Herd immunity is the indirect protection from an infectious disease that arises when a sufficiently high proportion of a population is immune, so that the chain of transmission is interrupted and even non-immune individuals are protected. It is a population-level concept, not an individual one.
Read →Levels of prevention
What are the levels of prevention?
The levels of prevention are a public-health framework that classifies preventive action by when it intervenes in the course of disease: primary prevention before disease begins, secondary prevention to detect it early, and tertiary prevention to limit its impact once established. Primordial prevention is sometimes added as an earlier stage.
Read →Risk assessment
What is epidemiological risk assessment?
Risk assessment is the structured process of estimating the probability and severity of harm to a population from a hazard. In public health it combines hazard identification, exposure assessment and risk characterisation to inform decisions about protecting health.
Read →Incidence & prevalence
What are incidence and prevalence?
Incidence and prevalence are the two fundamental measures of disease frequency. Incidence counts new cases arising in a population over a period; prevalence counts all existing cases at a point or over a period. The distinction between new and existing cases is central to epidemiology.
Read →Case fatality rate
What is the case fatality rate?
The case fatality rate is the proportion of people diagnosed with a particular disease who die from it within a given period. It measures the severity of a disease among recognised cases, and is distinct from the infection fatality rate and from population mortality rates.
Read →Mortality rate
What is a mortality rate?
A mortality rate is a measure of the number of deaths in a population over a period, relative to the size of that population. Forms include the crude death rate, age-specific and cause-specific rates, and age-standardised rates that allow fair comparison between populations.
Read →Morbidity
What is morbidity?
Morbidity is the presence of illness, disease or poor health in a population, and the burden it imposes. It is measured by how common conditions are and how much disability or impairment they cause, and is distinct from mortality, which concerns death.
Read →Relative risk
What is relative risk?
Relative risk, or risk ratio, is the ratio of the probability of an outcome in an exposed group to the probability in an unexposed group. It is a core measure of association in cohort studies, showing how many times more (or less) likely an outcome is with exposure.
Read →Odds ratio
What is an odds ratio?
An odds ratio is a measure of association comparing the odds of an outcome (or exposure) between two groups. It is the natural effect measure for case-control studies and, when the outcome is rare, approximates the relative risk.
Read →Cohort study
What is a cohort study?
A cohort study is an observational design that follows a group of people, classified by their exposure, over time to see who develops an outcome. Because it observes new cases as they arise, it can measure incidence and estimate relative risk.
Read →Case-control study
What is a case-control study?
A case-control study is an observational design that starts from the outcome: it compares people who have a condition (cases) with people who do not (controls), looking back at their past exposures. It is efficient for rare outcomes and yields an odds ratio.
Read →Selection bias
What is selection bias?
Selection bias is a systematic error that arises when the people included in a study, or retained in it, differ from the population they are meant to represent in ways that distort the association between exposure and outcome. It is a threat to validity, not a problem solved by larger samples.
Read →Confounding
What is confounding?
Confounding is a distortion of the association between an exposure and an outcome caused by a third variable that is related to both. A confounder can create the appearance of an effect, hide a real one, or change its apparent size, threatening causal interpretation.
Read →Basic reproduction number (R0)
What is the basic reproduction number?
The basic reproduction number, R0, is the average number of new infections caused by one infectious individual in a population that is entirely susceptible. It is a fundamental measure of how transmissible an infectious agent is, and it shapes thresholds for population immunity.
Read →Pandemic
What is a pandemic?
A pandemic is an epidemic that has spread across multiple countries or continents, affecting a large number of people. It sits at the top of a ladder of terms — outbreak, epidemic, pandemic — that describe disease occurring above the level normally expected, distinguished mainly by geographic scale.
Read →Life expectancy
What is life expectancy?
Life expectancy is the average number of years a person can expect to live, calculated from the death rates of a population using a life table. It is a key demographic indicator of population health, and is reported in period and cohort forms.
Read →Endemic
What is an endemic disease?
Endemic describes the constant or usual presence of a disease or infectious agent within a given population or geographic area. An endemic disease circulates at a relatively stable, expected baseline level rather than rising sharply, distinguishing it from an epidemic or pandemic.
Read →Outbreak
What is a disease outbreak?
An outbreak is the occurrence of cases of a disease in excess of what is normally expected in a population or area. It carries the same meaning as an epidemic but usually refers to a more limited geographic area or a shorter period, and is judged against the endemic baseline.
Read →Disease surveillance
What is disease surveillance?
Disease surveillance is the ongoing, systematic collection, analysis and interpretation of health data, tied to the timely dissemination of that information to those who can act on it. It underpins the detection of outbreaks and the planning, monitoring and evaluation of public-health practice.
Read →Attack rate
What is an attack rate?
The attack rate is the proportion of an at-risk population that develops a disease during a defined period, typically an outbreak. It is a form of cumulative incidence applied to a specific exposed group, widely used to compare risk between those exposed and not exposed to a suspected source.
Read →Recall bias
What is recall bias?
Recall bias is a systematic error that arises when participants in a study remember or report past exposures with differing accuracy or completeness. It is a particular concern in case-control and other retrospective studies, where information about exposure depends on memory of events before the disease developed.
Read →Information bias
What is information bias?
Information bias is a systematic error arising from inaccurate measurement or classification of exposure, outcome or other variables in a study. Also called measurement or misclassification bias, it concerns how data are obtained from those in the study, as distinct from who is selected into it.
Read →Screening
What is screening?
Screening, as a public-health concept, is the application of a test to apparently healthy people to identify those more likely to have a disease, so it can be addressed earlier. This page explains the methodology — sensitivity, specificity, lead-time and over-diagnosis — and not any individual screening decision.
Read →Ecological study
What is an ecological study?
An ecological study is an observational design that uses population-level, aggregate data rather than information on individuals. Each unit of analysis is a group — such as a country or region — and its great pitfall is the ecological fallacy: wrongly inferring individual-level relationships from group-level associations.
Read →Common questions
Epidemiology FAQ
What is epidemiology?+
Epidemiology is the study of how diseases and health-related states are distributed in populations and what factors influence that distribution. It provides the measures (such as incidence and prevalence) and study designs (such as cohort and case-control studies) used to investigate the causes and patterns of health and disease.
What is the difference between incidence and prevalence?+
Incidence counts new cases arising in a population over a period of time, while prevalence counts all existing cases at a point in time. Incidence measures the rate at which people develop a condition; prevalence measures how widespread it already is.
What is the difference between relative risk and an odds ratio?+
Relative risk is the ratio of the risk of an outcome in an exposed group to that in an unexposed group, and is calculated from cohort studies. An odds ratio compares the odds of exposure and is the measure used in case-control studies; it approximates relative risk when the outcome is rare.
Is this a medical or clinical resource?+
No. These pages are a research-methods and population-health reference that define epidemiological measures, study designs and public-health concepts. They are not medical advice and do not cover diagnosis, treatment or screening decisions.
How does this relate to CASRAI standards?+
CASRAI is a research-standards body. Epidemiological research generates large datasets whose value depends on transparent reporting (for example the STROBE guideline) and good research-data management — the same standards layer CASRAI maintains for the wider research ecosystem.
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