Epidemiology Cheat Sheet

The core ideas of Epidemiology distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

Incidence and Prevalence

Incidence measures the rate of new cases of a disease arising in a population over a specified time period, while prevalence measures the total number of existing cases at a given point or over a period. Together, these two fundamental measures describe the burden and dynamics of disease in a population.

Relative Risk

Relative risk (risk ratio) compares the probability of an outcome in an exposed group to the probability of the same outcome in an unexposed group. A relative risk greater than 1 indicates increased risk, while a value less than 1 indicates a protective effect. It is the primary measure of association used in cohort studies.

Odds Ratio

The odds ratio compares the odds of exposure among cases to the odds of exposure among controls, and it is the primary measure of association in case-control studies. When a disease is rare in the population, the odds ratio closely approximates the relative risk. It is especially useful when studying diseases with long latency periods.

Confounding

Confounding occurs when a third variable is associated with both the exposure and the outcome, distorting the apparent relationship between them. If not controlled for, confounders can make an association appear stronger, weaker, or even reversed. Epidemiologists address confounding through study design (randomization, restriction, matching) and analysis (stratification, multivariable regression).

Cohort Study

A cohort study follows a group of individuals who differ in their exposure status over time to compare the incidence of outcomes between exposed and unexposed groups. Prospective cohorts follow participants forward in time, while retrospective cohorts use historical records. Cohort studies can establish temporal sequence and calculate incidence rates directly.

Case-Control Study

A case-control study starts by identifying individuals with a disease (cases) and comparable individuals without the disease (controls), then looks backward to compare their prior exposures. This design is efficient for studying rare diseases and diseases with long latency periods. The measure of association is the odds ratio.

Bias in Epidemiologic Studies

Bias is any systematic error in the design, conduct, or analysis of a study that leads to an incorrect estimate of the association between exposure and outcome. The two major categories are selection bias, where the study sample does not represent the target population, and information bias, where exposure or outcome data are measured inaccurately. Recognizing and minimizing bias is central to producing valid epidemiologic evidence.

Herd Immunity

Herd immunity occurs when a sufficient proportion of a population is immune to an infectious disease, either through vaccination or prior infection, so that the chain of transmission is disrupted and even non-immune individuals are indirectly protected. The threshold for herd immunity varies by disease and depends on the basic reproduction number (R0).

Screening and Surveillance

Screening is the systematic application of a test to identify individuals with a disease or risk factor before symptoms appear, allowing for early intervention. Surveillance is the ongoing, systematic collection, analysis, and interpretation of health data essential for planning, implementing, and evaluating public health practice. Both are core tools of applied epidemiology.

Bradford Hill Criteria

The Bradford Hill criteria are a set of nine principles proposed by Sir Austin Bradford Hill in 1965 to evaluate whether an observed association between an exposure and a disease is causal. The criteria include strength, consistency, specificity, temporality, biological gradient, plausibility, coherence, experiment, and analogy. Temporality is the only criterion considered absolutely necessary.

Key Terms at a Glance

Incidence:The number or rate of new cases of a disease occurring in a specified population during a defined time period.

Prevalence:The total number of existing cases of a disease or condition in a population at a specific point or period in time, including both new and pre-existing cases.

Epidemic:The occurrence of disease cases in a community or region in excess of what is normally expected for that time and place.

Pandemic:An epidemic occurring over a very wide area, typically crossing international boundaries and affecting a large number of people across multiple continents.

Endemic:The habitual, constant presence of a disease or infectious agent within a given geographic area or population group at a baseline level.

Relative Risk:The ratio of the probability of an event occurring in an exposed group to the probability in a non-exposed group, used as the primary measure of association in cohort studies.

Odds Ratio:The ratio of the odds of exposure in the case group to the odds of exposure in the control group, used as the primary measure of association in case-control studies.

Confounding:A mixing of effects where a third variable, associated with both the exposure and the outcome, distorts the observed association between them.

Selection Bias:A systematic error in a study resulting from the way participants are selected or retained, such that the study population does not accurately represent the target population.

Information Bias:Systematic error due to inaccurate measurement or classification of exposure, outcome, or other study variables, including recall bias and misclassification.

Cohort Study:An observational analytic study in which a defined group of individuals is followed over time to determine the incidence of an outcome, comparing those exposed and unexposed to a factor of interest.

Case-Control Study:An observational analytic study that compares individuals with a specific disease (cases) to individuals without the disease (controls) with respect to prior exposures.

Randomized Controlled Trial:An experimental study design in which participants are randomly allocated to receive either the intervention or a control condition, minimizing confounding and providing strong evidence for causation.

R0 (Basic Reproduction Number):The expected number of secondary cases produced by a single infection in a completely susceptible population, quantifying the transmissibility of an infectious agent.

Herd Immunity:A form of indirect protection from infectious disease that occurs when a sufficient proportion of a population has become immune, thereby reducing the likelihood of infection for susceptible individuals.

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