CM7.1-11 | Epidemiology Methods — Glossary

Statistical procedure to remove the confounding effect of age structure when comparing crude death rates across populations with different age distributions; may be direct (applying study rates to standard population) or indirect (SMR method).

Transmission via droplet nuclei (<5 µm) that remain suspended in air for prolonged periods and travel distances >1 metre; requires adequate ventilation for prevention.

The proportion of an exposed group that develops illness during an outbreak; technically a proportion (not a rate) used for point-source or limited-exposure outbreaks.

The absolute difference in incidence rates between exposed and unexposed groups; represents the excess risk attributable to the exposure.

A living organism (usually an arthropod) in which the infectious agent undergoes replication or a necessary developmental stage before being transmitted to a new host.

Eight criteria (including strength, consistency, temporality, biological gradient, plausibility) for evaluating whether an observed association is causal; temporality is the only necessary criterion.

An infected person (or animal) that harbours a specific infectious agent without discernible clinical disease and serves as a potential source of infection for others.

A set of standard clinical, laboratory, and epidemiological criteria used to classify whether a person has the disease under investigation; may be confirmed, probable, or suspect.

The proportion of persons with a specified disease or condition who die from that disease within a specified period; numerator = deaths from disease, denominator = total cases of that disease.

An analytical observational study that selects persons with disease (cases) and without disease (controls) and compares their past exposure histories; yields Odds Ratio (OR).

India's statutory mechanism for mandatory registration of births and deaths under the Registration of Births and Deaths Act, 1969; primary source of vital statistics.

An analytical observational study that follows disease-free individuals (divided by exposure status) forward in time to compare the incidence of outcomes; yields Relative Risk (RR).

A disease caused by a specific biological agent (or its toxic products) that arises through transmission from an infected person, animal, or inanimate reservoir to a susceptible host.

The interval during which an infectious person or animal can transmit the agent directly or indirectly to others; may overlap with or precede the incubation period.

The range of values within which the true population parameter is estimated to lie with a specified probability (typically 95%); for ratios (RR, OR), a 95% CI excluding 1.0 indicates statistical significance at p<0.05.

A distortion of the true exposure-outcome association caused by a third variable that is associated with both the exposure and the outcome, and is not an intermediate in the causal pathway.

Consolidated Standards of Reporting Trials — a 25-item checklist for evaluating the completeness of reporting of randomised controlled trials.

The systematic evaluation of a research study's validity (is the result true?), results (how large is the effect?), and applicability (can the findings be applied to this population?).

Total deaths from all causes in a population per 1,000 mid-year population per year; not adjusted for age structure.

The use of data generated by digital systems — electronic health records, social media, internet search trends, GPS, participatory health apps — to study disease distribution, conduct surveillance, and inform public health decisions.

Transmission via large respiratory droplets (>5 µm) expelled during coughing, sneezing, or speaking, which travel ≤1–2 metres and settle out of the air relatively quickly.

The incorrect inference that an association observed at the group (aggregate) level also holds at the individual level; a fundamental limitation of ecological study designs.

The occurrence of a disease in a community or region in excess of what is normally expected for that place and time; an outbreak is an equivalent term typically used for more localised events.

A histogram plotting the number of new cases on the Y-axis against date/time of symptom onset on the X-axis; the shape reveals the transmission pattern (point-source, propagated, or mixed).

Conceptual model of communicable disease as an interaction among host (susceptible individual), agent (causative factor), and environment (external conditions mediating contact).

The study of distribution and determinants of health-related states in specified populations, and the application of this study to the control of health problems (Last's Dictionary / Park).

The extent to which a study's findings can be generalised from the study population to other populations or settings; also called generalisability or applicability.

Software that integrates epidemiological data with geographic maps to enable spatial visualisation, disease mapping, cluster detection, and geographic analysis of health outcomes.

The indirect protection of susceptible individuals in a population when a sufficient proportion of the population is immune, preventing sustained chains of transmission.

The proportion of a population that must be immune to prevent endemic transmission; calculated as 1 − 1/R₀, where R₀ is the basic reproduction number.

The rate at which new cases of a disease occur in a population at risk during a defined time period; numerator = new cases, denominator = population at risk × time.

The time interval between exposure to an infectious agent and the appearance of the first signs or symptoms of the resulting disease.

Deaths of infants under one year of age per 1,000 live births in the same year; a key indicator of child health and socioeconomic development.

India's national disease surveillance network using three reporting forms (P/L/S) for community syndromic reporting, laboratory confirmation, and clinical case data to enable rapid outbreak detection.

The physical separation of confirmed infectious cases from susceptible persons for the duration of the communicable period to prevent transmission.

Maternal deaths (related to pregnancy or within 42 days of its termination) per 100,000 live births; denominator is live births, not all pregnancies.

A living organism that passively carries a pathogen on its body (usually legs or body surface) without the pathogen undergoing replication or development.

The course of a disease from initial pathogenic exposure (pre-pathogenesis) through subclinical disease, clinical disease, and outcome (recovery, disability, death) in the absence of medical intervention.

The probability that a person with a negative test result truly does not have the disease; = TN / (TN + FN) × 100; higher prevalence → lower NPV.

Deaths in the first 28 days of life per 1,000 live births; reflects birth-related causes (asphyxia, prematurity, neonatal infection).

National Family Health Survey — a nationally representative household survey providing data on reproductive, maternal, child health, nutrition, and increasingly non-communicable disease indicators.

A disease that physicians and health facilities are legally required to report to public health authorities; in India, regulated centrally for internationally notifiable diseases and by state acts for others.

The statistical hypothesis that assumes no association between the study variables; statistical tests evaluate the probability of observing the data if H₀ is true (the p-value).

The number of patients who must receive an intervention to prevent one additional adverse outcome; NNT = 1 / absolute risk reduction; a measure of clinical significance complementing the relative risk.

The ratio of the odds of exposure among cases to the odds of exposure among controls in a case-control study; OR = ad/bc from the 2×2 table; approximates RR when disease is rare (<5-10%).

The probability of observing the data (or more extreme results) if the null hypothesis is true; p<0.05 is the conventional threshold for statistical significance — does not indicate clinical importance.

The proportion of a population that had the disease at any time during a defined period; includes cases present at the start plus incident cases during the period.

The three descriptive axes of epidemiology: person (who is affected), place (where cases occur), and time (when — secular trends, seasonality, epidemic curves).

The proportion of a defined population that has the disease at a specific point in time; includes all existing cases (new and old).

An epidemic in which all cases are exposed to a common source at approximately the same time; the epidemic curve shows a single bell-shaped peak spanning ≤1 incubation period.

The excess incidence in the entire population (compared to the unexposed) attributable to the exposure; accounts for both the RR and the prevalence of the exposure in the population.

The probability that a person with a positive test result truly has the disease; = TP / (TP + FP) × 100; depends on disease prevalence (higher prevalence → higher PPV).

An epidemic in which disease spreads from person to person; the epidemic curve shows multiple irregular peaks, each separated by approximately one incubation period.

The restriction of the movement of exposed but not-yet-symptomatic persons during the maximum incubation period of the disease to prevent possible transmission if they become infectious.

The condition under which the Odds Ratio approximates the Relative Risk; applies when disease incidence is low (<5–10%) so that case and control series represent the exposed and unexposed populations proportionally.

A type of information bias in which cases and controls differ in the accuracy or completeness with which they recall past exposures, typically because cases are more motivated to search for explanations.

The ratio of incidence of disease in the exposed group to incidence in the unexposed group; computed from cohort studies; RR=1 = no association, RR>1 = risk factor, RR<1 = protective.

The alternative hypothesis — the researcher's expected finding — states a direction of association between exposure and outcome. The null hypothesis (H₀) assumes no association.

The natural habitat in which an infectious agent lives, multiplies, and on which it depends for survival; may be human, animal, or environmental.

A dual-record demographic surveillance system operated by the Registrar General of India to produce reliable estimates of birth rates, death rates, and IMR at national and state levels.

A statistical computation performed before data collection to determine the minimum number of study participants needed to detect an effect of a specified size with a given confidence level and power.

The presumptive identification of unrecognised disease or defect in apparently healthy individuals by means of tests, examinations, or other procedures that can be rapidly applied.

The proportion of susceptible contacts of a primary case who develop illness within the maximum incubation period; measures transmissibility within close contact groups.

Systematic error arising when study participants are not representative of the target population, or when the process of selection is related to both the exposure and the outcome.

The proportion of truly diseased persons correctly identified as positive by the test; = TP / (TP + FN) × 100; a high-sensitivity test minimises false negatives (SnNout).

Surveillance conducted at selected strategic sites (sentinel sites) to detect trends in specific diseases (e.g. HIV in ANC clinics, influenza in ILI/SARI sites) at lower cost than universal surveillance.

The specific person, animal, object, or substance from which an infectious agent passes to a host, directly or indirectly through a vehicle or vector.

The proportion of truly non-diseased persons correctly identified as negative by the test; = TN / (TN + FP) × 100; a high-specificity test minimises false positives (SpPin).

Ratio of observed deaths in a study population to expected deaths calculated by applying reference age-specific rates to the study population's age structure; expressed as observed/expected × 100.

Strengthening the Reporting of Observational Studies in Epidemiology — a 22-item checklist for evaluating the reporting of cohort, case-control, and cross-sectional studies.

The practice of combining multiple independent data sources to estimate a health parameter that no single source can measure reliably alone.

Deaths in children under 5 years per 1,000 live births; captures both infant and 1–4 year mortality.

A structured interview method used with family members after a death to assign probable cause of death when medical certification was absent; improves CRS cause-of-death validity in rural settings.

A conceptual model for complex, multi-factorial disease causation (especially NCDs) in which many interacting factors contribute to disease, in contrast to the single-agent epidemiological triad.

Ten WHO criteria (1968) for evaluating whether a disease and test are suitable for a mass screening programme; includes requirements for the disease to be serious, have a detectable latent stage, and have effective treatment available.

An infection transmissible between animals and humans under natural conditions; examples include rabies, plague, leptospirosis, and avian influenza.