Topic

Epidemiology and risk measures

Epidemiology describes how often disease occurs and how much risk changes with exposure or treatment. Measures include incidence, prevalence, relative risk and number needed to treat, along with diagnostic test measures such as sensitivity and positive predictive value that feed into screening models.

Concepts in this topic

• Absolute RiskThe probability that an individual in a defined population will experience a specific outcome over a given period, without reference to a comparison.
• Absolute Risk ReductionAbsolute risk reduction is the arithmetic difference in the probability of a specified adverse outcome between a comparator group and an intervention group over the same defined time horizon.
• Additive Interaction ScaleThe additive interaction scale judges two exposures to interact when their joint risk difference departs from the sum of their separate risk differences.
• All-Cause MortalityAll-cause mortality is the rate of death from any cause in a population; models take background rates from life tables, and trials use it as an endpoint.
• Analytic EpidemiologyAnalytic epidemiology tests causes of disease by comparing groups, estimating the relative risks and attributable fractions used in economic models.
• Antimicrobial ResistanceAntimicrobial resistance (AMR) is the ability of microbes to withstand antimicrobial medicines, making infections harder to treat and costlier to manage.
• Attack RateAttack rate is the proportion of an at-risk population that develops the disease during an outbreak or other defined period: a risk, not a true rate.
• Attack Rate RatioA measure comparing the attack rate of a disease between two groups within an outbreak, such as those exposed and unexposed to a source.
• Attributable FractionThe attributable fraction is the share of cases, in the exposed or a whole population, that would not occur without an exposure, used to apportion burden.
• Attributable RiskAttributable risk is the risk in exposed minus unexposed people: the excess attributed to exposure, which health economic models turn into costs and QALYs.
• Basic Reproduction NumberThe basic reproduction number (R0) is the average number of infections one typical case causes in a fully susceptible population, used in vaccine models.
• Bayes TheoremBayes theorem is the rule that updates a prior probability with new evidence to give a posterior probability, used for diagnostic tests and model inputs.
• Case Fatality RateCase fatality rate (CFR) is the proportion of people diagnosed with a disease who die of it within a defined period, a measure of severity.
• Cumulative IncidenceCumulative incidence is the probability that a person initially at risk experiences a specified new event within a defined time interval.
• Diabetes ComplicationsLong-term health consequences of poorly controlled diabetes, including cardiovascular disease, kidney disease, nerve damage, and eye disease.
• Effective Reproduction NumberThe average number of secondary infections from one infected person in a population not entirely susceptible, accounting for existing immunity or control measures.
• Hazard RatioA hazard ratio (HR) is the ratio of the event rates (hazards) in two groups, the usual measure of a treatment effect on survival in trials and HTA models.
• Healthy Life ExpectancyA population health measure expressing the average years an individual is expected to live in good health, adjusting for time in disability.
• Herd Immunity ThresholdThe proportion of a population that must be immune to achieve herd immunity, mathematically related to a pathogen's basic reproduction number.
• IncidenceThe number of new cases of a disease occurring within a defined population over a specified period, unlike prevalence, which counts existing cases.
• Incidence RateA measure of disease frequency calculated as new cases divided by total person-time at risk observed, accounting for varying follow-up lengths.
• Incidence Rate RatioA measure comparing the incidence rate of an outcome between two groups, accounting for differences in total person-time at risk.
• Life ExpectancyThe expected number of additional years lived from a specified age under a stated set of mortality rates.
• Lifetime PrevalenceThe proportion of a population that has experienced a condition at any point in life up to assessment, unlike point prevalence.
• Likelihood RatioA ratio comparing the likelihood of the same observed evidence under two specified conditions or statistical models.
• Mortality RateA mortality rate measures deaths in a defined population over a specified period relative to the corresponding population or person-time at risk, with its scale and denominator stated explicitly.
• Negative Likelihood RatioA diagnostic test measure expressing how much a negative result changes the odds a patient does not have the condition tested for.
• Negative Predictive ValueThe probability that an individual with a negative diagnostic test result truly does not have the condition being tested for.
• Number Needed to HarmThe number of patients who would need exposure to a treatment or risk factor for one additional patient to experience an adverse outcome.
• Number Needed to ScreenThe number of individuals who would need screening for a condition to prevent one adverse outcome, based on prevalence and programme effectiveness.
• Number Needed to TreatThe reciprocal of a nonzero absolute difference in event risk between an intervention and comparator, interpreted as the expected number treated for one additional beneficial or harmful outcome over a stated period.
• Odds RatioA measure comparing the odds of an outcome in one group to the odds in another, common in case-control studies and logistic regression.
• Period PrevalenceThe proportion of a population with a particular condition at any point during a specified period, combining existing and newly developed cases.
• Point PrevalenceThe proportion of a population found to have a particular condition at a single specific point in time, unlike period prevalence or incidence.
• Population Attributable FractionThe proportion of disease cases in an entire population that would not occur if a specific exposure were eliminated, accounting for its prevalence.
• Population Attributable RiskThe reduction in disease incidence within a population that would result from eliminating a harmful exposure entirely, expressed as an absolute rate.
• Positive Likelihood RatioA diagnostic test measure expressing how much a positive result increases the odds that a patient truly has the condition tested for.
• Positive Predictive ValueThe probability that an individual with a positive diagnostic test result truly has the condition being tested for.
• Post-Test ProbabilityThe updated probability that a patient has a condition after a diagnostic test result is known, combining pre-test probability with the likelihood ratio.
• Posterior OddsThe updated odds that a hypothesis is true after incorporating new evidence, found by multiplying prior odds by the likelihood ratio.
• Pre-Test ProbabilityThe estimated probability that a patient has a condition before a diagnostic test result is known, based on prevalence and clinical presentation.
• PrevalenceThe total proportion of a population with a particular condition at a given time, covering both new and pre-existing cases.
• Prevalence DifferenceThe absolute difference in the prevalence of a condition between two groups, giving a direct measure of disparity in disease burden.
• Prevalence RatioThe prevalence ratio (PR) is the prevalence of a condition in an exposed group divided by that in an unexposed group, used to compare disease burden.
• Prevented FractionThe proportion of disease cases avoided in a population due to a protective exposure or intervention, the counterpart to the attributable fraction.
• Prior OddsThe odds that a hypothesis is true before incorporating new evidence, the starting point for Bayesian updating alongside a likelihood ratio.
• Rate DifferenceThe absolute difference between the incidence rates in two groups, giving a direct measure of the excess rate of disease from an exposure.
• Rate RatioA measure comparing the incidence rates of an outcome between two groups, accounting for differences in person-time at risk.
• Relative RiskRelative risk compares the probability of an event in one group with the probability in another group over a defined period.
• Relative Risk ReductionThe proportional reduction in risk associated with a treatment, calculated as one minus the relative risk, expressed as a percentage.
• Reproduction NumberA general term for the average number of secondary infections one infected individual produces, covering both the basic and effective reproduction numbers.
• Risk DifferenceThe absolute difference between the risk of an outcome in one group and in another, the excess or reduced risk from an exposure.
• Risk RatioA measure calculated as the ratio of risk in one group compared to another, synonymous with relative risk for event comparisons.
• Secondary Attack RateA measure of infectious disease transmission, the proportion of susceptible close contacts of a primary case who themselves become infected.
• SensitivityThe proportion of people with a specified target condition who test positive at a stated diagnostic threshold, calculated as true positives divided by true positives plus false negatives.
• SpecificityThe proportion of people without a specified target condition who test negative at a stated diagnostic threshold, calculated as true negatives divided by true negatives plus false positives.
• Vaccine EffectivenessThe degree to which a vaccine reduces disease risk under real-world conditions in a general population, unlike efficacy measured under trial conditions.
• Vaccine EfficacyThe degree to which a vaccine reduces disease risk among vaccinated individuals under the controlled conditions of a clinical trial.
• Years of Life LostYears of life lost (YLL) measure premature mortality as deaths times standard remaining life expectancy at the age of death, the fatal part of the DALY.