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A DICTIONARY OF EPIDEMIOLOGY SECOND EDITION Edited for the International Eprdemiological A.uocialion bT John M.. Last TW9C7.TsEZo71 W New York Oxford Toronto OXFORD UNIVERSITY PRESS 1988

Oxford University Press Oaford New York Toronto Delhi Bombay Calcutta Madrar Karachi Pcial'n6JaTa Singapore Hong Kong Tokyo Nairobi Dar ea Salaam Cape Torn Melbourne Auckland and asaociaied companies in Berhn Ibadan C.opyright © 1988 by International Epidemiological Association, Inc. Published bv Oxford Univenitv Press. Inc., 200 Madison Avenue. New 1'ork. New York 10016 O.ford u a repstered irademark of Oxford University Preu All riRhu rearrsed. No pan of this publiritinn mav be reproduced. stored in a rrtneval rvrem, or tranamitied. in any form or bv anr means, ek_rironic. mechanical. phoaoropvinR. recording. or aherwire, without she prior permission of (hford Univenhy Presr. Libnrv of Congress GauloRinR-in-Publication Data A Dictionacy of epidemioloKr. Includes b:h/io_Rraphies. I. Epidemiob6v-Dictionaries. I. tast. John M.. 1926- 11. International EpideminloRiral Arrociation. -- IDNLM: 1. EpidemioloRv-dictionaricr. WA 13 D5551 RA651.D55 1988 611.4'0)'21 B7-_l1109 ISBN 0-19-505400-6 15BN 0-19-505181--! tpbk.l 59CZ15t?'0z 2416111091551 Printcd in the Uniied Sutes of America on acid-free paper Foreword The International Epidemiological Association is extremely pleased that the Dicteonnry ojEpedcmiologY has been so successful that a second edition has been demanded. As one of the Asso- ciation's aims is to "spread the message," this work is an exam- ple of "what we call it." Only if we all understand the same thing when a particular term is used will the aim of the Asso- ciation be capable of being fulfilled. This dictionary is funda- mental to this objective. W. W. Holland, MD FRCGP FRCP FFGM President, lnternational Epidemiological Association

Preface gsM1sCzoz This dictionary, ictionary, appearing now in its second edition, is an at- tempt to bring some order to the occasionally chaotic nomen- clature of epidemiology. It is intended for all who are inter- ested in epidemiology, especially those who are beginning to study the subject, those whose first language is not English, and those from other fields who need to know the terms epide- miologists use. Like all rapidly expanding sciences, epidemiology has been confounded by the proliferation of words and phrases to de- sc-ribe its co_ nccpts, principles, methods, and procedures. The creation of new terms and disagreement about the meaning of old ones can confuse beginners and established epidemiologists alike. Remarks by users of the first edition have reinforced the view that the boundaries should be wide rather than narrow, that the language should be simple, that some terms many epide- miologists think everyone already knows should be included. The second edition is larger than the first, partly for this rea- son, and because terms omitted from the first edition have been included and many old entries expanded. The dictionary is not an index of permitted and proscribed usage. I hope that it is authoritative without being authoritar- ian. Where synonyms exist, the definition appears under the most commonly used of these, but preference for one term over another is not necessarily implied. In a few instances, the use of a term is deprecated. Some terms that are properly de- scribed as slang or jargon have been included because they are widely used and their meaning is not always clear from the con- text. Murphy's description of jargon is worth recalling: "ob- scure and/or pretentious language, circumlocutions, invented meanings, and pomposity delighted in for its own sake." There was disagreement among the contributors to this edi- tion about including certain acronyms and eponyms. An acro- nym is a word made up of letters from two or more other words, e.g. ANOVA for analysis of variance, or from initial letters, e.g.

preface viii WHO for World Health Organization. All lay and technical vo- cabularies contain acronyms; epidemiology has its fair share. By convention, acronyms are spelt out the first time they ap- pear in a text, and, if they are numerous, considerate editors sometimes supply a glossary, oc at least list the acronyms along with the words for which they stand in an index. Although this dictionary is not the place for extensive mention of acronyms, a few appeared in the first edition, and a few more appear here. Eponyms, the attachment of personal or place names to con- cepts, diseases, methods or specific studies, also occur often enough in published papers and books for us to recognize that beginners need some guidance to the meaning of those most widely used. Some appeared in the first edition, and a few have been added to the second-though again this dictionary is not the proper place for a full glossary of epidemiological eponyms (where would such a glossary end!). As was the case with the first edition, a large number of epi- demiologists from many countries ntries have participated in this re- vision. The original modest notices in a couple of journals and a few casual remarks_ among friends produced a mailing list of some forty persons, mainly in North America and the United Kingdom. The mailing list rapidly grew until, by the fifth round of correspondence in December 1986, there were 108 corre- spondents in 25 countries. The list continued to grow after this fifth and final round; but the published roster of names that follows this preface is both more and less than the number of active participants. Some seemingly inquired just from curiosity and played no further part. Others wrote lengthy and often vigorously argumentative comments and suggestions express- ing not only their own views but those of colleagues in their academic department or institution-in one instance,, col- leagues elsewhere in that nation. In addition to extensive comments from these correspon- dents, I have made good use of other technical dictionaries and glossaries in compiling this revision. All of these are listed in the bibiliography, and many are also to_ be found in footnotes that follow specific entries. The compilers of dictionaries must exercise the greatest care in the choice of words and in their arrangement. Most entries in this dictionary have been repeatedly discussed with many contributors, and in nearly all instances the wording has been agreed upon by all; on the rare occasions when agreement eluded us, the final decision was mine alone. Therefore, I accept full responsibility for the deficiencies in the finished product. The work has been sponsored by the International Epide- 49C?;TSC2'0% ix miological Association, which provided partial travel support for me to attend two meetings in 1986; further support was provided by the McLean Foundation and the Milbank Memo- rial Fund. All royalties from the sale of this edition, like those from the first edition, will go to the International Epidemiol- ogical Association. Finally, I thank Jeffrey House of Oxford University Press for helpful advice and encouragement. Ottawn, Canada November 1987 J. M. L. preface

Contributing Editors J. H. ARRAMCON Jerusalem. Israel URSULA ACKERMAN-LIERRICH Basel, Switzerland RORERT ALLARD Montreal, Quebec. Canada JOHN C. BAILAR IH Washington, DC, USA CHRISTOP-HER 13ALDOCK brisbane. Queensland, Australia ROlERTO G. ISARUZZI Sao Paulo, Brazil ARRAM S. BENENSON San Diego, CA, USA ROGER BERNARD Geneva, Switzerland JEAN-FRANCOtS BOIVIN Montreal, Quebec, Canada BERNARO J. BRARIN Madang, Papua New Guinea C. RALPH BUNCHER Cincinnati, OH, USA BEVERLEY CARLSON New York, NY, USA JAMES CHIN Berkeley, CA, USA MICHEL COLEMAN Oxford, England L. CAYOLLA DA MOTTA Lisbon, Portugal GARET'H DAVIES New Ilaw. Surrey. England RICHARD DICKER Atlanta, GA, USA ALVAN R. FEINSTEIN New Haven, CT, USA DAVID FINNEY Edinburgh. Scotland JOSEPH L. FLEI55 New York, NY. USA GARY D. FRIEDMAN Oakland, CA. USA MICHAEL GARRAWAY Edinburgh, Scotland SANDER GREENLAND Los Angeles, CA. USA TEE GUIDOTTt Edmonton. Alberaa, Canada WALTER W. HOLIAND London, England B_ARRARA HULKA Chapel Hill. NC, USA MICHEL IRRAHIM Chapel lfill, NC. USA LESUE M. IRwlc Sydney. NSW, Australia MILOS JENICEK Montreal, Quebec. Canada L. KARHAUSEN Luxembourg, Luxembourg

contributing editors HENK LAMRERTS Amsterdam, the Netherlands JOHN M. LAST Ottawa, Ontario. Canada DALE LAWRENCE Atlanu. GA. USA DAVID E. LILIENFELD New York. NY. USA GENEvICVE LOSLIER Hudson, Quebec, Canada ROSERT MACLENNAN Brisbane, Queensland, Australia MARGARET F. MCCANN Chapel Hill, NC. USA ANTHONY B. MILLER Toronto. Ontario, Canada KIUMARSS NASSERI Teheran, Iran JOHN S. NEURERCER Kansas City, KA, USA NORMAN D. NOAH London. England ROBERT OSEASOHN San Antonio, TX, USA HARRIS PASTIDES Amherst. MA, USA MIQUEL PORTA Barcelona. Spain DAVID RoetNsoN GeneYa, Switzerland GEOrrREY A. ROSE London. England xii KENNETH J. ROTHMAN Boston, MA, USA JAMES J. SCHLFSSELMAN Bethesda, MD, USA B.A.SOUTHGATE London, England CLAUDE STROHMENGER Ottawa. Ontario, Canada IAN St1THERLAND Cambridge, England MERVYN SUSSER New York, NY, USA A.V. SWAN London, England RODOLio SARACCI Lyon, France MICHEL TIIURIAUx Copenhagen, Denmark B. TOMA Maisons-Alfort. France CARL TYLER Atlanta. GA, USA ROBERT B. WALLACE Iowa City. IA, USA STEPHEN D. WALTER Hamilton, Ontario, Canada KERR L. WHITE Stanardsville, VA, USA DONALD Wlc_tE Ottawa, Ontario, Canada CORRESPONDING EDITORS ERIK ALLANDER Huddinge, Sweden ALBERTA ALZATE Cali. Colombia GEOrrREY A. ANDERSON Ottawa. Ontario, Canada MARY JANE ASHLEY Toronto. Ontario, Canada r R.S. BHOPAL Glasgow, Scotland PATRICIA A. BUrr1ER Houston. TX, USA ARVIND A. CARPENT€R Oak Ridge. TN, USA CARL J. CASPERSON Atlanta, GA. USA DENIS CHARPIN Marseilles, France GERALD ER_ALD R. CHASE Denver. CO, USA EMIL E. CRISTOEANO Akron, OH. USA ORESTES FAGET CEPERO Havana. Cuba ANNE HERSEY COULiON Los Angeles. CA. USA DAVID CUNDIrT Trenton, NJ, USA ROGER DETEtS Los Angeles, CA, USA ROnGER DOYLE Buffalo.. NY, USA GERARD DURo15 Panis, France JACtjUELINE FARIA Quebec City, Quebec. Canada G.1. FoaREs Edinburgh, Scotland LINA FORCtER Sydney, NSW. Australia EDUARDO_ FRANCO Sao Paulo, Brazil JACK FROOM Stony Brook, NY. USA TRUt.S GEDDE-DA/IL Oslo, Norway O.N.GIU. London, England xsn correaponding editors DAVID GOLDSMITH Edmonton, Alberta, Canada PATRICIA GRAVGS Madang, Papua New Guinea VINCENT GUINEE Houston, TX, USA MATTI HAItAMA Tampere, Finland A. SCOTT HENDERSON Canberra. ACT, Australia CATHERINE HILL Villejui(, France ERNEST B. HOOK Albany. NY, USA J. JANCAR Bristol, England FINN KAMPER-JORGENSEN Copenhagen, Denmark H.W. KANIS l.elysud, the Netherlands JENNIrER KELSEY New York, NY, USA MARY CLAIRE KING Berkeley. CA, USA MAUa1CE KING Leeds. England GEORGE KNOx Birmingham. England JESS KRAUs Los Angeles, CA. USA H. OWVER LANCASTER Sydney, NSW, Australia F.D.K. LtoDEU. Montreal, Quebec. Canada IAN MCDOWEt1. Ottawa. Ontario, Canada H. MICHAEL MAETz Birmingham, AL. USA Luls MACJIo Lisbon, Portugal ssCzTsCzo%

corresponding editon J.S. MALHI Brighton. England DIttEP V. MAVALANRER Ahmedabad, India DAVID MORRIS London, England C.S. MUIR Lyon, France ENRIQUE N,(JERA Seville, Spain KATE O'CONNOR Ottawa. Ontario, Canada H. TUNSTAt.L PEDOE Dundee, Scotland 1. PLESKO Bratislava, Czechoslovakia PAULA RANTIKALLIO Oulu, Finland R.A. RoelNSON St. Paul, MN. USA ROGER ROCHAT Atlanta, GA, USA JEFFREY ROSEMAN Birmingham, AL, USA PHILIP ROSS Honolulu, H1, USA M. SARIC Z.2breb, Yugoslavia BjORN SMEDOY Uppsala, Sweden ANDREU SE('.URA Barcelona, Spain xiv COLIN SOSKOLNE Edmonton, Alberu, Canada PAULA STEWART Ottawa, Ontario, Canada RORERT SPASOFF Ottawa, Onurio, Canada D.STRACHILOV Soha, Bulgaria HuGH TILSON Research Triangle Park, NC, USA TOSHIO TOYAMA Tokyo,Japan EDWARD J. TRAPIDo Miami, FL. USA STJEPAN VIDACEK 7agreb, Yugoslavia ANNE WALLING Wichita, KA, USA RORERT WE3T Cardiff, Wales JAN WIENPAHL Los Angeles, CA, USA WU X1-KE Hefei, Anhui, China M.J. WYSOCKI Warsaw, Poland FARAT YUSUF North Rydc, NSW, Australia FRED ZERFAS Los Angeles, CA. USA SHUx1AN ZU Hefei, Anhui. China A Dictionary of Epidemiology o4MzsCzoZ

UEZZ SC07:o% t AsoRnDN RATE The estimated annual number of abortions per 1000 women of repro• ductive age (usually defined as age 15-44). ABORTION RAT70 The estimated number of abortions per 100 live births in a given year. AtSCIbSA The distance along the horizonul coordinate or x axis, of a point P from the vertical or Y axis of a graph. PJee also AXIS, GRAPH, ORDINATE. AR,iOLUTE RISK Usuall?this term means the observed or calculated risk of an event in a population under study, as contrasted with the relative risk. Sometimes, however, it is a synonym for attributable fracuon, excess risk, or risk difference; because of the inconsistency, this term should be avoided. See also RISK. ACCEPTARLE RISK The risk that has minimal detrimental effects, or for which the ben- elits outweigh the potential hazards. Epidemiologic study has provided data for calculation of risks associated with many medical procedures and also with nccupa- tional and environmental exposures; these data are used, for instance, in CLlNICAL DECISION ANALYSIS. ACaURwt:v The degree to which a measurement, or an estimate based on measure- ments, represents the true value of the attribute that is being measured. See also MEASUR[MENT-, PRORLEMS WITH TERMINOLOGY. AtJQUAINTANCE NETWORK Group of persons in contact or communication among whom transmission of an infectious agent and of knowledge, attitudes, and values is pos- sible, and whose social interaction may have health implications. See also TRANSMIS- SION Or INFECTION. ACQVIRED IMMUNODEF7ClENCY ><VNDROMC (Syn: acquired immune deficiency syndrome) (AIDS) For surveillance purposes, the Centers for Disease Control, Allanu, Geor- gia,' define a case of AIDS as an illness characterized by (1) one or more of a group of opportunistic or indicator diseases that are indicative of underlying cellular im- munodcficiency; (2) absence of all known underlying causes of cellular immuno- deficienc-y and absence of all other causes of reduced resistance to opportunistic or indicator diseases. Additional criteria are serum positive for HIV antibody, positive cuhure for HIV, and reduction of T4 "helper" lymphocytes. The opportunistic or indicator diseases associated with AIDS include certain pro- tozoal and helminth infections, notably Pneweocyitu cnrinti pneumonia and toxo- plasmosis; fungal infections, notably candidiasis of esophagus. Irachea, bronchi or lungs and cryptococcosis, especially affecting the central nervous syste m; bacterial infections, notably with certain mycobacleria; viral infections, notably cytomegalo, viruw and herpes simplex; and cancer, notably Kaposi4 sarcoma and lymphoma limited to the brain. AIDS-related complex (ARC) is the combination of H I V positive test with lymph.

ADL scale 4 adenopathy and persistent low fever but without immunodeFiciency or opportunis- tic diseases. ' I9R7 Revision of case definition of AIDS for surveillance purposes. MMWR 36, I5:4S-9S. 1987. ACTIVITIES Or DAILY LtVING (ADL) SCALE A scale devised by Katz and others' to score physical JIrilitv/disability; used to measure outcomes of interventions for various chronic disabling conditions such as arthritis. The scale is based on scores for re- sponses to questions about mobiliq•, selftare, grooming, etc. This was the first widely used scale of this type; othen, mostly refinements or variations of the ADL L snle, have since been developed. ' Kau S. Ford. AB. Moskowitz. RN', Jackson. BA, Jaffe. M W: Studies of illness in the aged. The index of ADL, a sundardi:ed measure of biological function. fAAfA 185:9f4-919. 1963. ACTUARIAL RATE See FORCE OF MORTALITI'. ACTUARIAL TA/LE See_ LIFE TARLE. ACUTE 1. Referring to a health effect, brief; sometimes loosely used to mean severe. 2. Referring to exposure. brief, intense, or short-term; sometimes specifically re- ferring to brief exposure of high intensity. See also CHRONIC. ADAPTATION A heritable component of the phenotype which confers an advantage in survival and reproductive success._ The process by which organisms adapt to envi- ronmenul conditions. ADDIT7VE MODEL A model in which the combined effect of several factors is the suwn of V the effects that would be produced by each of the factors in the absence of the others. For example. if factor X adds x.c7r to risk in the absence of f', and if factor Y adds YSli to risk in the absence of X, an additive model states that the two factors together will add (x+r)g to risk. See also INTERACTION; LINEAR MODEL; MATHEMAT- ICAL MODEL; MULTIPLICATIVE MqDEL.ADJUSTMENT A summarizing procedure for a statistical measure in which the effects of differences in composition of the populations being compared have been mini- mized by statistinl methods. Examples are adjustment by regression analysis and by standardization. Adjustment often is performed on rates or relative risks, com- monly because of differing age distributions in populations that are being com- pared. The mathematical procedure commonly used to adjust rates for age differ- ences is direct or indirect standardization. ADVERSE REACTION, SIDE EFF'ECT Any undesirable or unwanted consequence of a pre- ventive, diagnostic, or therapeutic procedure. AETIOI.OGY, AETIOLOCIC See ETIOLOGY, ETIOLOGIC. 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 (al least one of which must be present), or a complex of two or more factors whose combined presence is essential for thrdevelopment of the disease. See also CAUSnu-T-Y; NECESSARY AND SUFFICIENT CAUSE. ACE-PERIOD COHORT ANALYSIS See COHORT ANALYSIS. AGE-SEx PYRAMID See POPULATION PYRAMID. AGE-SEx_ REGISTER List of all clients or patients of a medical practice or s_ervice, classi- fied by age (birthdate) and sex; provides denominator for calculating age- and sex- specific rates. AGE-SPECIFIC FERTILtTY RATE The number of births occurring during a specified pe- 5 aealytic study riod 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 births to women of the speci- fied age is usually divided by the midyear population of women of that age. ACE-SPEGIFIC RATE A nte for a specified age group. The numerator and denominator refer to the same age group. Example: Age-specific death rate (age Y5-34) Number of deaths among residents age 25-34 in an area in a year x 100,000 Average (or midyear) population age 25-34 in the area in that year The multiplier (usually 100,000 or 1.000,000) is chosen to produce a rate that can be expressed as a convenient number. ACE STANDARDIZATION A procedure for adjusting rates, e.g. death rates, designed to minimize the effects of differences in age composition when comparing rates for dl(ferenl populations. See also ADJUSTMENT, STANDARDIZATION. AGCREGATION S1AS (Syn: ecological bias) See ECOLOGICAL FALLACY. AGING OF THE PQPOLAT7ON A demographic term, meaning an increase over time in the proportion of older persons in the population. It does not necessarih imply an increase in life expectancy or that "people are living longer than they used to." The principal determinant of aging in the population has been a decline in the birth rate: when fewer children are born than in prior vears, the result, in the absence of a rise in the death rate at higher ages. has been an increase in the proportion of older persons in the population. In developed societies, however. mortalitY change is becoming a factor: little further mortality reduction can occur in thc lirsl hall of life, so reductions are beginning to occur in the third and fourth quarters of lile, leading 10 a rise in the proportion of older persons from this cause. AIRBORNE INFECTION A mechanism of transmission of an infectious agenl bY particles, dust, or DROPLET NUCLEI suspended in the air. See als0 TRANSMI3SION OF INFECTION. ALGORrTHM 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 conlmonly used to describe a structured process, for insunce, relating to computer program- ming or to health planning. See also DECISION TREE. IrAt.GORmtM, CUNICAL (Syn: clinical protocol) An explicit description rsf sleps 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 epidemiologic 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 chromosrnne. ALPHA ERROR See ERROR. TYPE 1. ALPHA LEVEL See SIGNIFICANCE LEVEL ,~NALYSIS OF VARIANCE A statistical technique that isolates and assesses the contribution of categorical independent variables 10 Yariation in the mean of a continuous de- pendent variable. The observations are classified according to their calegories 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. J NALYTIC STUDY A study designed to examine associations- colnmonly putative or hy- polhr.,ized causal relationships. An analytic study is usually concerned with idrnli- R.M4,VII /'SC[..0Z

anirn.f model 6 fying or measuring the effects of risk factors, or is concerned with the health effects of specific exposure(s). Contrast descriptive study, which does not test hypotheses. The common types of analytic study are CROSS-SECTIONAL, COHORT, and cASE-GON- TROL. In an analytic study, individuals in the study population may be classified according to absence or presence (or future development) of specific disease and according to "attributes" that may influence disease occurrence. Attributes may in- clude age, race, sex, other disease(s), geneuc, biochemical, and physiological char- acteristics, economic status, occupation, residence, and various aspects of the envi- ronment or personal behavior. See 2130 _ CASE CONTROL STUDY; COHORT STUDY; GROSS- SECTIONAL S_TVDY; STUDY DESIGN. ANIMAL MODEL Study in a population of laboratory animals that uses conditions of an- imals analogous to conditions of humans to model processes comparable to those that occur r in human populations. See a1s0 ExrERtMENTAL EPIDEMIOLOGY. ANTAGONISM Opposite o_ f SYNERGtsM. The situation in which the combined effect of two or more factors is smaller than the solitary effect of any one of the factors. In elOAssAY, the term may he used to refer to the situation when a specified response is produced by exposure to either of two factors but_ not by exposure to both to- gether. ANTTttaoroMETRY The technique that deals with the measurement of the size, weight, and proportions of the human body. ANrsrwororxtuc (adj.) Pertaining to an insect's preference for feeding on humans even when nonhuman hosts are available. ANTI/ODY Protein molecule formed by 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 specifically to the foreign substance (antigen) that elicited its production, thus supplying a mechanism for protection against infectious diseases. Antibody is epidemiologically important because its con- centration (titer) can be measured in individuals, and, therefore, in populations_ . See also SEROErIDEMiOIAGY. ANTtGEN 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. ANTICENIC DRIR This term describes the "evolutionary" changes that uke place in the molecular structure of DNA/RNA in micro-organisms 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.' It leads to alteration (usually slow and progressive) in the antigenic composition, and thus in the immunologic re- sponses of individuals and populations to exposure to the micro-organisms con- cerned. See also ANTIGE_NIC SHIrT. ' Palcse P. 1'oung JF: Variation of Influenza A. B, and C Viruses. Seuwr 215:1468-1473. 1982. ANTIGENIC sHt" This term describes mutation, i.e.. a sudden change in molecular structure of DNA/RNA in micro-organisms, especially viruses, which 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 influenza A virus associated with large-scale epidemic and pandemic spread. Antigenic shift is responsibk for the susceptibility of host populations to a new strain of influenza virus. See als0 ANTICENIC DRIrT. ANTICENICI7Y (Syn: immunogenicity) The ability of agent(s) to produce a systemic or a local immunologic reaction in the host. - , 7 u.ociation, a.rmmetrical Asuovtrtus A group of taxonomically diverse animal viruses that are unified by an ep- idemiologic concept, i.e., transmission between vertebrate host organisms_ by blood- feeding (hematophagous) arthropod vectors such as mosquitoes, ticks, sand flies, and midges. The term is a contraction of arthropod-borne virus. The intenction of arbovirus, vertebrate host(s), and arthropod vector gives this class of infections several unique epidemiologic features. See VECTOR-s_ORNE INEE6- TION for definition of terms used to describe these features. AREA 6AMrL_INC A method of sampling that can be used when the numbers in the pop- ulation 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, then a second stage of sampling may be conducted. - ARUTHMEI7C MEAN The sum of all the values in a set of ineasuremenu, divided by the number of values in the set. AR17F7CIAL 1NrELLIGENCE A branch of computer science in which attempts are made to V duplicate human intellectual functions. One application is in diagnosis, in which computer programs are often based upon epidemiologic analyses of data in hospital charts or other clinical records. ASCERTAWMt7NT Tfle process of determining what is happening in a population or study group, e.g., famih and household composition, occurrence of cases of specific dis- eases: the latter is also known as case-finding. ASCERTAINMENT s1A5 Systematic failure to represent equally 211 classes of nses or per- sons 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 influenced by culture, custom, or idiosvncracy; or, for example, in genetic studies, from the statistical chance of selecting from large or small fami- lies. ASSAY The quantitative or qualitative evaluation of a hazardous substance; ; the results of such an evaluation. Asso_CSwnON (Syn: correlation, (statistialJ dependence, relationship) Sutistical depen- dence between two or more events, characteristics, or other variables. An associa- tion is present if the probability of occurrence of an event or characteristic, or the quantity of a variable. depends upon 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 the occurrence of higher values of a variable is associated with the occurrence of higher values of another variable. In a negative association, the occurrence of higher values of one variable is associated with lower values of the other variaMe. An as- sociation may be fortuitous or may be produced by various other circumstances; the presence of an association does not necessarily imply a causal relationship. If the use of the term "association"is confined 10 situations in which the relationship between two variables is statistically significant, the terms "statistical association" and "sutistically significant association" become tautological. However, ordinary usage is seldom so precise as this. The terms "association" and "relationshili" are often used interchangeably. Associations can be broadly grouped under two headings, symmetrical or non- causal (see below) and asymmetrical or causal. Ai6OCtATION, ASYMMETRICAL (Syn: asymmetrical relationship) The definitive conditions of asymmetrical associations are direction and time. Independent variable X must cause changes in dependent variable Y, and the "caucal" va6at.h m,.-r n..._.a- ?1. C4.C?'TSCzOZ