ABATEMENT The process of reducing or minimizing public health dangers and nui- sances, usually supported by regulation or legislation; e.g., noise abatement, pollution abatement.

ABC APPROACH “Abstinence, Be faithful, use Condoms.” ABC strategies are promoted to combat, foremost, infection with HIV and the HIV/AIDS pandemic as well as other sexually transmitted diseases. These are pragmatic sex education policies that aim at balancing abstinence-only sex education by including education about safe sex and birth control methods. Excessive emphasis on ABC strategies may marginalize broader, integrated programs in which all components are mutually reinforcing. See also CNN approach.

ABORTION RATE The estimated annual number of abortions per 1000 women of repro- ductive age (usually defined as ages 15–44).

ABORTION RATIO The estimated number of abortions per 100 live births in a given year.

ABSCISSA The distance along the horizontal coordinate, or x axis, of a point P from the vertical or y axis of a graph. See also axis; graph; ordinate.

ABSOLUTE EFFECT The effect of an exposure (expressed as the difference between rates, proportions, means), of the outcome, etc., as opposed to the ratio of these meas- ures.12 See also risk difference.

ABSOLUTE POVERTY LEVEL Income level below which a minimum nutritionally adequate diet plus essential nonfood requirements is not affordable.13 The amount of income a person, family, or group needs to purchase an absolute amount of the basic necessities of life. See also relative poverty level.

ABSOLUTE RATE The number of specified health events (disease onset, death, etc.) divided by the time at risk in a defined population over a specified time interval.12 See also event rate; rate ratio.

ABSOLUTE RISK (AR) The probability of an event (usually adverse, but it may also be beneficial) in the population under study. Contrast with relative risk. The number of events in a group divided by the total number of subjects in that group. Sometimes AR is wrongly used as a synonym for attributable fraction; excess risk; or risk difference.

ABSOLUTE RISK INCREASE (ARI) The absolute risk of adverse events in the treat- ment group (ART) minus the absolute risk of events in the control group (ARC): ARI = ART—ARC. Same as the risk difference. Also, the proportion of treated persons who experience an adverse event minus the proportion of untreated persons who experience the event. See also number needed to harm (NNH).


  1. The arithmetic difference between two event rates. The amount by which the risk of an undesirable event is reduced by elimination or control of a particular exposure. It enables an estimate of the number of people spared the consequences of an exposure.
  1. The absolute risk of events in the control group (ARC) minus the absolute risk of events in the treatment group (ART): ARR = ARC—ART. The negative of the risk difference. Also, the proportion of untreated persons who experience an adverse event minus the proportion of treated persons who experience this event.

The reciprocal of the ARR is the number needed to treat (NNT). The ARR is one measure of the strength of an association. It varies with the underlying risk of an event; e.g., it becomes smaller when event rates are low. The ARR is higher and the NNT lower in groups with higher absolute risks.14, 15 See also event rate; Hill’s criteria of causation; measure of association; probability of causation; relative risk reduction.

ACCEPTABLE RISK Risk that appears tolerable to some group. Risk that has minimal or long-term detrimental effects or for which the benefits outweigh the potential haz- ards. Epidemiological research has provided data for calculation of risks increased by many medical procedures as well as by occupational and environmental exposures; these data are used, for instance, in clinical decision analysis and health technology assessment.

ACCEPTANCE SAMPLING (Syn: stop-or-go sampling) Sampling method that requires division of the “universe” population into groups or batches as they pass a specified time point (e.g., age), followed by sampling of individuals within the sampled groups.

ACCIDENT An unanticipated event—commonly leading to injury or other harm—in traffic, the workplace, or a domestic or recreational setting. The primary event in a sequence that leads ultimately to injury if that event is genuinely not predictable. Epi- demiological studies have demonstrated that the risk of accidents is often predictable and that accidents are preventable. This word is preferably avoided in many types of scientific works.

ACCUMULATION OF RISK The extent of cumulative damage to biological systems as the number, duration, or severity of exposures increases and as body systems age and become less able to repair damage. The notion that life course exposures or insults gradually accumulate through episodes of illness and injury, adverse environmental conditions, and health damaging behaviors. Exposures increasing risk of disease may be independent or clustered; in the latter case an accumulation model with risk clustering is used.16 See also developmental and life course epidemiology; thrifty phenotype hypothesis.


  1. The degree to which a measurement or an estimate based on measurements represents the true value of the attribute that is being measured. Relative lack of error. See also measurement, terminology of; validity, study.
  1. The ability of a diagnostic test to correctly classify the presence or absence of the target disorder.The diagnostic accuracy of a test is usually expressed by its sensitivity and specificity.

ACE American College of Epidemiology.

ACQUAINTANCE NETWORK A group of persons in contact or communication among whom transmission of an infectious agent and of knowledge, behavior, and values is possible and whose social interaction may have health implications. See also context; transmission of infection.

ACQUIRED IMMUNODEFICIENCY SYNDROME (AIDS) (Syn: acquired immune defi- ciency syndrome) The late clinical stage of infection with human immunodeficiency virus (HIV), recognized as a distinct syndrome in 1981. The opportunistic or indica- tor diseases associated with AIDS include certain protozoan and helminth infections, fungal infections, bacterial infections, viral infections, and some types of cancer. The role of AIDS as an indicator in surveillance has diminished since the advent of highly active antiretroviral therapy (HAART). See also human immunodeficiency virus (hiv).

ACTIVE LIFE EXPECTANCY See disability-free life expectancy.

ACTIVITIES OF DAILY LIVING (ADL) SCALE A scale devised by Katz and others17 to score physical ability/disability; used to measure outcomes of interventions for various chronic, disabling conditions, such as arthritis. The scale is based on scores for responses to ques- tions about mobility, self-care, grooming, etc. This was the first widely used scale of this type; others, mostly refinements or variations of the ADL scale, have since been developed.

ACTIVITY SETTING The places, events, routines, and patterns that structure the experi- ence of everyday life; e.g., a classroom, a neighborhood resident meeting, a commuter train, family meals, a waiting room in a hospital. The unit by which culture and community are propagated across time.18 See also behavior setting; context.

ACTUARIAL RATE See force of mortality.

ACTUARIAL TABLE See life table.


  1. Referring to a health effect: sudden onset, often brief; sometimes loosely used to mean severe.
  1. Referring to an exposure: brief, intense, or short-term; sometimes specifically referring to a brief exposure of high intensity. See also chronic.


  1. The process by which organisms surmount environmental challenges. See also resilience.
  1. A heritable component of the phenotype that confers an advantage in survival and reproductive success.

ADDITIVE MODEL A model in which the combined effect on the risks or rates of several factors is the sum of the effects that would be produced by each of the factors in the absence of the others. For example, if factor X adds x to risk in the absence of Y and factor Y adds y to risk in the absence of X, an additive model states that the two factors together will add (x + y) to risk. See also interaction; linear model; mathematical model; multiplicative model.

ADELF Association des Épidémiologistes de Langue Française (Association of Epidemi- ologists of French Language).

ADHERENCE Health-related behavior that adheres to the recommendations of a doctor, other health care provider, or investigator in a research project. The word adherence aims to avoid the authoritarian associations of compliance, formerly used to describe this behavior. Concordance is another alternative to compliance.19

ADJUSTMENT A summarizing procedure for a statistical measure in which the effects of differences in composition of the populations being compared have been minimized by statistical methods.20 Examples are adjustment by regression analysis, by inverse-probability weighting, and by standardization. Adjustment is often performed on a rate or on an effect measure, commonly because of differing age and sex distributions in the populations being compared. The mathematical procedure commonly used to adjust rates for age differences is (direct or indirect) standardization.

ADULT LITERACY RATE The percentage of persons 15 years of age and over who can read and write.21

ADVERSE REACTION An undesirable or unwanted consequence of a preventive, diag- nostic, or therapeutic procedure. See also side effect.

AES American Epidemiological Society.
AGE The WHO recommends that age should be defined by completed units of time, counting the day of birth as zero.

AGE DEPENDENCY RATIO See dependency ratio.

AGENT (OF DISEASE) A factor—such as a microorganism, chemical substance, or form of radiation—whose presence, excessive presence, or (in deficiency diseases) relative absence is essential for the occurrence of a disease. A disease may have a single agent, a number of independent alternative agents (at least one of which must be present), or a complex of two or more factors whose combined presence is essential for the develop- ment of the disease. See also causality; necessary cause.


AGE-SEX PYRAMID See population pyramid.

AGE-SEX REGISTER A list of all clients or patients of a medical practice or service, classified by age (birthdate) and sex; it provides denominators for calculating age- and sex-specific rates.

AGE-SPECIFIC FERTILITY RATE The number of live births occurring during a specified period to women of a specified age group divided by the number of person-years lived during that period by women of that age group. When an age-specific fertility rate is calculated for a calendar year, the number of live births to women of the specified age is usually divided by the midyear population of women of that age.

AGE-SPECIFIC RATE A rate for a specified age group. The numerator and denominator refer to the same age group.

AGE STANDARDIZATION A procedure for adjusting rates (e.g., death rates) designed to minimize the effects of differences in age composition in comparing rates for different populations. See also adjustment; standardization.

AGGREGATION BIAS (Syn: ecological bias) See aggregative fallacy; ecological fallacy; atomistic fallacy.

AGGREGATE SURVEILLANCE The surveillance of a disease or health event by collect- ing summary data on groups of cases (e.g., general practitioners taking part in surveil- lance schemes are asked to report the number of cases of specified diseases seen over a specified period of time).

AGGREGATIVE FALLACY An erroneous application to individuals of a causal relation- ship observed at the group level. A type of ecological fallacy (sometimes just a syno- nym) and an antonym of the atomistic fallacy.22

AGING OF THE POPULATION An increase over time in the proportion of older persons in a defined population. It does not necessarily imply an increase in life expectancy or that people are living longer than they used to. In the past, the principal cause of aging of populations has been a decline in the birth rate: in the absence of a rise in the death rate at higher ages, when fewer children are born than in prior years, the proportion of older persons in the population increases. Nowadays, in developed societies, little fur- ther mortality reduction can occur in the first parts of life; thus, reductions in mortality that occur in the third and fourth quarters of life are leading to a rise in the proportion of older persons. See also demographic transition.

AIRBORNE INFECTION An infection whose agent is transmitted by particles, dust, or droplet nuclei suspended in the air. The infective agent may be transmitted by a patient or carrier in airborne droplets expelled during coughing and sneezing. See also transmission of infection.

ALGORITHM Any systematic process that consists of an ordered sequence of steps with each step depending on the outcome of the previous one. The term is commonly used to describe a structured process—for instance, relating to computer programming or health planning. See also decision tree.

ALGORITHM, CLINICAL (Syn: clinical protocol) An explicit description of steps to be taken in patient care in specified circumstances. This approach makes use of branching logic and of all pertinent data, both about the patient and from epidemiological and other sources, to arrive at decisions that yield maximum benefit and minimum risk.

ALLELE Alternative forms of a gene occupying the same locus on a chromosome. Each of the different states found at a polymorphic site.23

ALLOCATION BIAS An error in the estimate of an effect caused by failure to implement valid procedures for random allocation of subjects to intervention and control groups in a clinical trial.

ALLOCATION CONCEALMENT A method of generating a sequence that ensures ran- dom allocation between two or more arms of a study without revealing this to study subjects or researchers. The quality of allocation concealment is enhanced by compu- ter-based random allocation and other procedures to make the process impervious to allocation bias. See also blind(ed) study; random allocation.

ALMA-ATA DECLARATION See health care; health for all; primary health care.

ALPHA ERROR See error, type i.

AMBIENT Surrounding; pertaining to the environment in which events are observed.

AMES TEST A bioassay for mutagenesis, using bacteria as target, to detect and screen for potentially carcinogenic compounds. Developed from the early 1970s by Bruce Ames and colleagues at the University of California, Berkeley. See carcinogen.

ANALYSIS OF VARIANCE (ANOVA) A statistical technique that isolates and assesses the contribution of categorical independent variables to the variance of the mean of a continuous dependent variable. The observations are classified according to their cate- gories for each of the independent variables, and the differences between the categories in their mean values on the dependent variable are estimated and tested for statistical significance.

ANALYTICAL STUDY A study designed to examine putative or hypothesized causal relationships; hence, most such studies can be conceptualized as etiological studies. An analytical study is usually concerned with identifying or measuring the effects of risk factors or with the health effects of specific exposure(s) or interventions. Contrast descriptive study, which usually does not test hypotheses. The common types of ana- lytical study are cross-sectional, cohort, and case-control. In an analytical study, individuals in the study population may be classified according to the absence or pres- ence (or future development) of specific disease and according to “attributes” that may influence disease occurrence. Attributes may include age; race; sex; other disease(s); genetic, biochemical, and physiological characteristics; social position, economic status; occupation; residence; and various aspects of the environment or personal behavior. See also research design.

ANECDOTAL EVIDENCE Evidence derived from descriptions of cases or events rather than systematically collected data that can be submitted to formal epidemiological and statistical analysis. Such evidence must be viewed with caution but sometimes is use- ful to raise a warning of danger or to generate hypotheses (e.g., as shown by voluntary reporting of adverse drug events). See also case reports.

ANEUGENIC An agent that affects cell processes and structures resulting in the loss or gain of whole chromosomes. See also mutagenic; clastogenic.

ANIMAL MODEL A study in a population of laboratory animals that uses conditions of animals analogous to conditions of humans to model processes comparable to those that occur in human populations. See also experimental epidemiology.

ANTAGONISM (Opposite:synergism)

  1. One of two types of effect modification or interaction: the effect modifier
    diminishes the effect of the putatively causal variable. The situation in which the combined effect of two or more factors is smaller than that expected from the effect of one factor in the absence of the other factors.24
  1. Inbioassay,thesituationwhenaspecifiedresponseisproducedbyexposuretoeither of two factors but not to both together. Antagonism exists if there are persons who will get the disease when exposed to one of the factors alone but not when exposed to both.

ANTHROPOMETRY The technique dealing with the measurement of the size, weight, and proportions of the human body.

ANTHROPOPHILIC (adj.) Pertaining to an insect’s preference for feeding on humans even when nonhuman hosts are available.

ANTIBODY Protein molecule produced in response to exposure to a “foreign” or extrane- ous substance (e.g., invading microorganisms responsible for infection) or active immu- nization. May also be present as a result of passive transfer from mother to infant, via immune globulin, etc. Antibody has the capacity to bind specifically to the foreign substance (antigen) that elicited its production, thus supplying a mechanism for protec- tion against infectious diseases. Antibody is epidemiologically important because its concentration (titer) can be measured in individuals and therefore, in populations. See also seroepidemiology.

ANTIGEN A substance (protein, polysaccharide, glycolipid, tissue transplant, etc.) that is capable of inducing specific immune response. Introduction of antigen may be by the invasion of infectious organisms, immunization, inhalation, ingestion, etc.

ANTIGENIC DRIFT The “evolutionary” changes that take place in the molecular struc- ture of DNA/RNA in microorganisms during their passage from one host to another. It may be due to recombination, deletion, or insertion of genes, to point mutations, or to several of these events. This process has been studied in common viruses, notably the influenza virus.25 It leads to alteration (usually slow and progressive) in the antigenic composition and thus in the immunological responses of individuals and populations to exposure to the microorganisms concerned. See also antigenic shift.

ANTIGENICITY (Syn: immunogenicity) The ability of agent(s) to produce a systemic or a local immunological reaction in the host.

ANTIGENIC SHIFT A mutation, or sudden change in molecular structure of DNA/RNA, in microorganisms, especially viruses, that produces new strains of the microorganism. Hosts previously exposed to other strains have little or no acquired immunity. Anti- genic shift is believed to be the explanation for the occurrence of strains of the influenza A virus associated with large-scale epidemic and pandemic spread. See also antigenic drift.

APACHE Acronym for Acute Physiology and Chronic Health Evaluation, a scoring system used to predict the outcome of critical illness or injury. This system and its variations (APACHE II, etc.) assign scores for state of consciousness, eye movements, reflexes, and physiological data such as blood pressure.26

APGAR SCORE A composite index used to evaluate neonatal status by assigning numer- ical scores (0–2) to heart rate, respiration, muscle tone, skin color, and response to stimulation. Developed by Virginia Apgar (1909–1974) a U.S. pediatrician/anesthetist. Low scores are associated with a poor prognosis.

APHA American Public Health Association.
APPLIED EPIDEMIOLOGY The application and evaluation of epidemiological knowledge and methods (e.g., in public health or in health care). It includes applications of etiological research, priority setting and evaluation of health programs, policies, technologies, and services. It is epidemiological practice aimed at protecting and/or improving the health of a defined population. It usually involves identifying and investigating health problems, monitoring changes in health status, and/or evaluating the outcomes of interventions. It is generally conducted in a time frame determined by the need to protect the health of an exposed population and an administrative context that results in public health action.27 See also field epidemiology; hospital epidemiology.

ARBOVIRUS An arthropod-borne virus. Various RNA viruses transmitted principally by arthropods, including the causative agents of encephalitis, yellow fever and dengue. A group of taxonomically diverse animal viruses that are unified by an epidemiological concept, i.e., transmission between vertebrate host organisms by bloodfeeding (hemat- ophagous) arthropod vectors such as mosquitoes, ticks, sand flies, and midges. The inter- action of arbovirus, vertebrate host, and arthropod vector gives this class of infections unique epidemiological features. See vector-borne infection for terms that describe these features.

ARCHIPELAGO (AR-kÉ™-PEL-É™-goh) sometimes called an island group or island chain, is a chain, cluster, or collection of islands, or sometimes a sea containing a small number of scattered islands.

AREA SAMPLING A method of sampling that can be used when the numbers in the population are unknown. The total area to be sampled is divided into subareas (e.g., by means of a grid that produces squares on a map); these subareas are then numbered and sampled using a table of random numbers. Depending upon circumstances, the population in the sampled areas may first be enumerated, and then a second stage of sampling may be conducted.

ARITHMETIC MEAN See mean, arithmetic; average.
ARM (of a trial) A group of persons whose outcome in a study is compared with that of another group or groups; commonly the arms of a trial are categorized as experimental and control groups.

ARMITAGE-DOLL MODEL A model of carcinogenesis in which time elapsed since exposure, not age, is a prime determinant of cancer.28 The model postulates three phases:

  1. A normal cell develops into a cancer cell after a small number of transition stages.
  1. Initially the number of normal cells at risk is very large, and for each cell transition is a rare event.
  1. The transitions are independent of each other. There are no presumptions about precipitating causes of the transition from normal to cancerous cell. Named for the statistician Peter Armitage (1924–) and the epidemiologist Richard Doll (1912– 2005).

ARTIFICIAL INTELLIGENCE A branch of computer science in which attempts are made to duplicate human intellectual functions. One application is in diagnosis, in which com- puter programs are based upon epidemiological analyses of data abstracted from clini- cal records.

ASCERTAINMENT The process of determining what is happening in a population or study group—e.g., family and household composition, occurrence of cases of specific diseases; the latter is also known as case finding.

ASCERTAINMENT BIAS Systematic failure to represent equally all classes of cases or persons supposed to be represented in a sample. This bias may arise because of the nature of the sources from which persons come (e.g., a specialized clinic); from a diag- nostic process influenced by culture, custom, or idiosyncracy; or, in genetic studies, from the statistical chance of selecting from large or small families.

ASSAY The quantitative or qualitative evaluation of a (hazardous) substance in water food, soil, air, etc; the results of such an evaluation. See also bioassay.

ASSOCIATION [Syn: (statistical) dependence, relationship; sometimes correlation is used synonymously.

1. Statistical dependence between two or more events, characteristics, or other variables. An association is present if the probability of occurrence of an event or characteristic, or the quantity of a variable, varies with the occurrence of one or more other events, the presence of one or more other characteristics, or the quantity of one or more other variables. The association between two variables is described as positive when higher values of a variable are associated with higher values of another variable. In a negative or inverse association, the occurrence of higher values of one variable is associated with lower values of the other variable. An association may be fortuitous or may be produced by various other circumstances; the presence of an association does not necessarily imply a causal relationship. In epidemiological and clinical research, the terms association and relationship may often be used interchangeably.

2. One of three properties David Hume, in his Treatise of Human Nature of 1739, deemed necessary (but insufficient standing alone) for assigning cause; the other two properties of a cause are connection and time order.9,10,71

ASSOCIATION, FORTUITOUS A relationship between two variables that occurs by chance and is thought to need no further explanation. See also random; correlation, nonsense.

ASSOCIATION, SPURIOUS An ambiguous term used with different meanings by different authors. It may refer to artifactual, fortuitous, false, secondary, or many kinds of noncausal associations owing to chance, bias, or confounding.

ASSORTATIVE MATING Selection of a mate with preference (or aversion) for a particu- lar genotype (i.e., nonrandom mating).

ASYMPTOTIC Pertaining to a limiting value, for example, of a dependent variable, when the independent variable approaches zero or infinity. See large sample method.

ASYMPTOTIC CURVE A curve that approaches but never reaches zero or infinity (e.g., an exponential or reciprocal exponential curve).

ASYMPTOTIC METHOD See large sample method.

ATOMISTIC FALLACY An erroneous inference about causal relationships in groups made on the basis of relationships observed in individuals.22 The counterpart of the ecological fallacy. An automatic, literal, or mechanical translation of causal infer- ences made in individuals to population groups may be wrong because different causal processes may operate when the individual is the unit of interest than when the unit is a population or group. The atomistic fallacy may occur when studies based on individuals (individual-level studies) are assumed to be valid and sufficient to make causal infer- ences at an upper level of aggregation (e.g., on the relationship between exposures and diseases at the group level). Relevant in particular when individual-level factors (e.g., income or gun ownership) and group-level factors (e.g., average income in the neigh- borhood or prevalence of gun holders in a city) capture or mediate different aspects of health risks. See also aggregative fallacy; ecological fallacy; individual thinking; multiple causation; population thinking.

ATTACK RATE The proportion of a group that experiences the outcome under study over a given period (e.g., the period of an epidemic). This “rate” can be determined empirically by identifying clinical cases and/or by means of seroepidemiology. It also applies in noninfectious settings (e.g., mass poisonings). Because its time dimension is uncertain or arbitrarily decided, it should probably not be described as a rate. See also infection rate; mass action principle; Reed-Frost model; secondary attack rate.

ATTENUATION Weakening (dilution) of the concentration, as of an antigen in a vaccine; also of an effect, e.g., relative risk.

ATTRIBUTABLE BENEFIT Antonym of attributable risk; a term that can be used when exposure is beneficial rather than harmful.

ATTRIBUTABLE FRACTION (Syn: attributable proportion) For a causal association, the proportion of the caseload that can be attributed to a particular exposure. It is the causal attributable difference (attributable risk) divided by the incidence rate in the group. It is the proportion by which the incidence rate would be reduced if the expo- sure were eliminated.29,30 The attributable fraction may apply to exposed individuals [attributable fraction (exposed)] or to the whole population [attributable fraction (population)].

ATTRIBUTABLE FRACTION (EXPOSED) [Syn: attributable proportion (exposed), attributable risk, etiological fraction (exposed), relative attributable risk]. With a given outcome, exposure factor, and population, the attributable fraction among the exposed is the proportion by which the incidence rate of the outcome among those exposed would be reduced if the exposure were eliminated. If there is no bias or confounding, it may be estimated by the formula

ATTRIBUTABLE FRACTION (POPULATION) [Syn: attributable proportion (popula- tion), etiological fraction (population), population attributable risk proportion, Levin’s attributable risk]. With a given outcome, exposure factor, and population, the attribut- able fraction for the population incidence rate is the proportion by which the incidence rate of the outcome in the entire population would be reduced if the exposure was eliminated. If there is no bias or confounding, it may be estimated by the formula

ATTRIBUTABLE PROPORTION See attributable fraction

ATTRIBUTABLE RATE, ATTRIBUTABLE RISK (Syn: causal rate difference, causal risk difference) The proportion of the rate (risk) of a disease or other outcome in exposed individuals that can be attributed to the exposure. This measure is estimated by subtracting the rate (risk) of the outcome (usually, incidence or mortality) among the unexposed from the rate (risk) among the exposed individuals; this estimate assumes that causes other than the one under investigation have had equal effects on the exposed and unexposed groups. Unfortunately, this term has been used to denote a number of different concepts, including the attributable fraction in the population, the attributable fraction among the exposed, the population excess rate, and the rate difference. See also absolute risk reduction; impact numbers.

ATTRIBUTABLE RISK (EXPOSED) This term has been used with different connotations to denote the attributable fraction among the exposed and the excess risk among the exposed. See also attributable fraction (exposed); rate difference.

ATTRIBUTABLE RISK PERCENT Attributable fraction expressed as a percentage of the total rate or risk rather than as a proportion.

ATTRIBUTABLE RISK PERCENT (EXPOSED) The attributable fraction among the exposed, expressed as a percentage of the total rate or risk among the exposed. See also attributable fraction (exposed).

ATTRIBUTABLE RISK PERCENT (POPULATION) The attributable fraction in the population, expressed as a percentage of the total rate or risk in the population. See also attributable fraction (population).

ATTRIBUTABLE RISK (POPULATION) This term has been used with different connotations to denote the attributable fraction in the population and the population excess risk. See also absolute risk reduction; attributable fraction (population); population excess rate.

ATTRIBUTE A qualitative characteristic of an individual or an item.

ATTRITION Reduction in the number of participants in a study as it progresses (i.e., during follow-up of a cohort). Losses may be due to withdrawals, dropouts, or protocol deviations.31 See also censoring.

ATTRITION BIAS A type of selection bias due to systematic differences between the study groups in the quantitative and qualitative characteristics of the processes of loss of their members during study conduct, i.e., due to attrition among subjects in the study. Different rates of losses to follow-up in the exposure groups may change the characteristics of these groups irrespective of the studied intervention.32,33


  1. An examination or review that establishes the extent to which a condition, process, or performance conforms to predetermined standards or criteria. Assessment or review of any aspect of health care to determine its quality; audits may be carried out on the provision of care, compliance with regulations, community response, completeness of records, etc.
  1. An evaluation of the quality of health care, the use of resources, and outcomes. See also health services research.
  1. The process of checking whether the accounts of an institution, company, or association are complete, accurate, and consistent; whether they agree with other records of activity; and whether they comply with legal requirements and professional standards.

AUSTRALIA ANTIGEN Hepatitis B surface antigen (HBsAg). So called because it was first identified in an Australian aborigine. HBsAg is a biomarker for the prevalence of infection with the virus of hepatitis B.


  1. In ethics, the principle of respect for human dignity and the right of individuals to decide things for themselves.
  1. In epidemiological practice and research, this principle is central to the concept of informed consent. It can conflict with the need to protect the population from identified risks (e.g., risks related to contagious disease) and with the need for access to personally identifiable health-related data and information. See also confidentiality; consent bias; privacy.

AUTOPSY DATA Data derived from autopsied deaths; used, for instance, to study aspects of the natural history of disease or trends in frequency of disease. Autopsies are done on nonrandomly selected persons; findings should therefore be generalized only with great caution. See also bias in autopsy series.

AUXILIARY HYPOTHESIS BIAS A form of rescue bias and thus of interpretive bias, which occurs in introducing ad hoc modifications to imply that an unanticipated finding would have occurred otherwise had the experimental conditions been different. Because experimental conditions can easily be altered in many ways, adjusting a hypothesis is a versatile tool for saving a cherished theory.34


  1. In science, loosely, the arithmetic mean. The arithmetic average of a set of n numbers is the sum of the numbers divided by n.
  1. A measure of location, either the mode or, in the case of numerical data, the median or the mean.
  1. Distribution of aggregate inequalities in a series among all the members of the series, so as to equalize them. See also measure of central tendency.
  1. In everyday speech, ordinary, usual, or normal; the normal or typical amount.

AVERAGE LIFE EXPECTANCY See expectation of life.


  1. One of the dimensions of a graph. A two-dimensional graph has two axes, the horizontal or x axis and the vertical or y axis. Mathematically, there may be more than two axes, and graphs are sometimes drawn with a third dimension. See also abscissa; ordinate.
  1. In nosology, an axis of classification is the conceptual framework (e.g., etiological, topographical, psychological, sociological). The International Classification of Diseases, for example, is multiaxial: the primary axis is topographical (i.e., body systems), while secondary axes relate to etiology, manifestations of disease, detail of sites affected, severity, etc.