Analysis of variance(ANOVA) ANALYSIS OF VARIANCE (ANOVA)A statistical technique that isolates and assesses the contribution of categorical independent variables to the variance of the mean 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 fluence 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 e 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 nimals 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 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 2. In BIOASSAY, the situation when a specified response is produced by exposure to either of two factors but not to both together. Antagonism exists if there are pe 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 insects 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
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 classifi ed according to their categories 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 signifi cance. 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 specifi c exposure(s) or interventions. Contrast descriptive study, which usually does not test hypotheses. The common types of analytical study are cross-sectional, cohort, and case-control. In an analytical study, individuals in the study population may be classifi ed according to the absence or presence (or future development) of specifi c disease and according to “attributes” that may infl uence 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 useful 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 modifi cation or interaction: the effect modifi er 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 2. In bioassay, the situation when a specifi ed response is produced by exposure to either 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 extraneous substance (e.g., invading microorganisms responsible for infection) or active immunization. May also be present as a result of passive transfer from mother to infant, via immune globulin, etc. Antibody has the capacity to bind specifi cally to the foreign substance (antigen) that elicited its production, thus supplying a mechanism for protection against infectious diseases. Antibody is epidemiologically important because its Analysis of variance (ANOVA) 6
7 Arbovirus 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. 2 It leads to alteration(usually slow and progressive) in the antigenic omposition 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 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 IL, etc. ) assign scores for state of consciousness, eye movements, reflexes, and physiological data such as blood pressure. Z6 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 timulation. 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 knowl or In of etiological research, priority setting and evaluation of health programs, policies, technologies, and s. 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. See also FIELD EPIDEMIOLOGY; HOSPITAL EPIDEMIOLOG ARBOVIRUS An arthropod-borne virus. Various RNA viruses transmitted principally ding 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- oophagous)arthropod vectors such as mosquitoes, ticks, sand fies, 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
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 specifi c 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 structure 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 infl uenza 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. Antigenic shift is believed to be the explanation for the occurrence of strains of the infl uenza 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, refl exes, and physiological data such as blood pressure.26 APGAR SCORE A composite index used to evaluate neonatal status by assigning numerical 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 defi ned 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 fi eld 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 unifi ed by an epidemiological concept, i.e., transmission between vertebrate host organisms by bloodfeeding (hematophagous) arthropod vectors such as mosquitoes, ticks, sand fl ies, and midges. The interaction 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. 7 Arbovirus
Area sampling 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 mpling 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 ARMITAGE-DOLL MODEL A model of carcinogenesis in which time elapsed since expo- a prime determinant of cancer. The model postulates three phases ormal cell develops into a cancer cell after a small number of transition stages. Initially the number of normal cells at risk is very large, and for each cell transition Is a rare event 3. 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 atistician 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 o ldy 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(.g, a specialized clinic): from a diag- nostic process influenced by culture, custom, or idiosyncracy or, in genetic studies, from he 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. J 1. Statistical dependence between two or more events, characteristics, or other variable 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 associated with values of a negative or inverse association, the occurrence of higher values of one variable associated with lower values of the other variable. An association may b 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
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 fi rst 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. 2. Initially the number of normal cells at risk is very large, and for each cell transition is a rare event. 3. 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 computer programs are based upon epidemiological analyses of data abstracted from clinical 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 specifi c diseases; the latter is also known as case fi nding. 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 diagnostic process infl uenced 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. Area sampling 8
Attributable fraction 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. .471 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 dif ferent 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 dependent 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 made on the basis of relationships observed in individuals. 2 The counterpar ECOLOGICAL FALLACY. An automatic literal or mechanical translation of caUSAL r of the 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(dilutionof 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. 2 30 The attributable fraction may apply to exposed individuals ATTRIBUTABLE FRACTION(EXPOSED)or to the whole population ATTRIBUTABLE FRACTION (POPULATION)
2. One of three properties David Hume, in his Treatise of Human Nature of 1739, deemed necessary (but insuffi cient 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 particular genotype (i.e., nonrandom mating). ASYMPTOTIC Pertaining to a limiting value, for example, of a dependent variable, when the independent variable approaches zero or infi nity. See large sample method. ASYMPTOTIC CURVE A curve that approaches but never reaches zero or infi nity (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 inferences 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 suffi cient to make causal inferences 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 neighborhood 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 benefi cial 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 exposure were eliminated.29,30 The attributable fraction may apply to exposed individuals [attributable fraction (exposed)] or to the whole population [attributable fraction (population)]. 9 Attributable fraction
Attributable fraction(exposed) 10 Exposed ttributable Incidence Exp Unexp Exp Unexp of the population that is exposed, and l-p the proportion not exposed. The lighter area represents cases that are attributable to the exposure(attributable risk, AR) and that would not have occurred in the absence of the exposure. It is assumed that in the absence of this hazardous exposure, the incidence would be I, in the whole population. From Spasoff R. 1999. with permission. ATTRIBUTABLE FRACTION(EXPOSED)[ Syn: attributable proportion (ex attributable risk, etiological fraction(exposed), relative attributable risk]. Wit. outcome,exposure factor, and population, the attributable fraction among the )nd 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 here I, is the incidence rate(proportion) among the exposed, I is the incidence rate (proportion) among the unexposed; or by the formula where RR is the rate ratio, I/I. It should not be confused with the ETIOLOGICAL FRACTION and the PROBABILITY OF CAUSATION ATTRIBUTABLE FRACTION(POPULATION) Syn: attributable proportion(popula tion), etiological fraction(population). population attributable risk proportion, Levin 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 wa eliminated. If there is no bias or confounding, it may be estimated by the formula
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 AF I I I e e u e = − where Ie is the incidence rate (proportion) among the exposed, Iu is the incidence rate (proportion) among the unexposed; or by the formula AF RR e = −1 RR where RR is the rate ratio, Ie / Iu . It should not be confused with the etiological fraction and the probability of causation. ATTRIBUTABLE FRACTION (POPULATION) [Syn: attributable proportion (population), etiological fraction (population), population attributable risk proportion, Levin’s attributable risk]. With a given outcome, exposure factor, and population, the attributable 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 Exposed Attributable Fraction Population Attributable Fraction IE IU Exp Unexp Exp Unexp p 1-p Incidence Attributable fractions. IE and IU are the incidence in the exposed and unexposed groups, p is the proportion of the population that is exposed, and 1-p the proportion not exposed. The lighter area represents cases that are attributable to the exposure (attributable risk, AR) and that would not have occurred in the absence of the exposure. It is assumed that in the absence of this hazardous exposure, the incidence would be IU in the whole population. From Spasoff R, 1999. With permission. Attributable fraction (exposed) 10