IARC International Agency for Research on Cancer. A WHO agency (

IATROGENESIS Literally, “doctor-generated”; often broadly used to refer to adverse effects of preventive, diagnostic, therapeutic, surgical, and other medical, sanitary and health procedures, interventions, or programs. The entire process through which a professional health activity generates an adverse health effect. There is a natural plurality of views on what constitutes iatrogenesis and its scope (e.g., clinical, social, and cultural iatrogenesis). Medicine and public health are obviously not the only professions that cause adverse effects: others are also part of some excessive professionalizing and

bureaucratization of contemporary societies.78,240 See also prevention, quaternary.

IATROGENIC EFFECT An adverse effect on health resulting from the activity of a health professional (e.g., a physician, a nurse, a professional of health promotion).

ICD See international classification of disease.

ICEBERG PHENOMENON That portion of disease which remains unrecorded or undetected despite physicians’ diagnostic endeavors and community disease surveillance pro- cedures is referred to as the “submerged portion of the iceberg.” Detected or diagnosed disease is the “tip of the iceberg.” The submerged portion comprises disease not medi- cally attended, medically attended but not accurately diagnosed, and diagnosed but not reported.241 Other terms have been proposed to describe this concept in parts of the world where icebergs are unknown (e.g., “ears of the hippopotamus,” “crocodile’s nose”).

ICHPPC See international classification of health problems in primary care.

ICIDH International Classification of Impairments, Disabilities, and Handicaps.

ICMJHE International Committee of Medical Journal Editors.

ICRC International Commission of the Red Cross, Red Crescent.

IDENTIFICATION NUMBER, IDENTIFYING NUMBER Unique number given to every individual at birth or at some other milestone. The Nordic and Baltic nations have a system based on a sequence of digits for birth date, sex, birthplace, and additional digits for each individual. Other systems (e.g., National Insurance number in the United Kingdom, Social Security number in the United States, and Social Insurance number in Canada) are some- times used but are neither universal nor unique, being sometimes applied to whole families or at least to more than one individual. See also hogben number; soundex code.

IDIOSYNCRASY A distinctive characteristic or peculiarity of an individual. In pharmaco- epidemiology, it means an abnormal reaction, sometimes genetically influenced, follow- ing the administration of a medication.

IEA International Epidemiological Association (, founded 1954. The aims of the IEA are to facilitate communication among those engaged in research and teaching in epidemiology throughout the world and to engage in the development and use of epidemiological methods in all fields of health including social, community, and preventive medicine and health services administration. These aims are achieved by holding scientific meetings and seminars, the publication of journals and reports, and by many other activities that take place every year on all continents. The official journal of the IEA is the International Journal of Epidemiology (

IEA-EEF International Epidemiological Association European Epidemiology Federation. The European Epidemiology Federation (EEF) is an official group within the IEA. All members of the IEA in Europe are automatically members of the EEF. Most national associations of epidemiologists in the European region are also part of the EEF.

ILLNESS See disease.

ILLNESS BEHAVIOR Conduct of persons in response to abnormal body signals. Such behavior influences the manner in which people monitor their bodies, define and inter- pret their symptoms, take remedial actions, and use the health care system. See also health behavior.

IMMUNITY, ACQUIRED Resistance acquired by a host as a result of previous exposure to a natural pathogen or foreign substance for the host, e.g., immunity to measles resulting from a prior infection with measles virus.

IMMUNITY, ACTIVE Resistance developed in response to stimulus by an antigen (infect- ing agent or vaccine) and usually characterized by the presence of antibody produced by the host.

IMMUNITY, NATURAL Species-determined inherent resistance to a disease agent, e.g., resistance of humans to the virus of canine distemper.

IMMUNITY, PASSIVE Immunity conferred by an antibody produced in another host and acquired naturally by an infant from its mother or artificially by administration of an antibody-containing preparation (antiserum or immune globulin).

IMMUNITY, SPECIFIC A state of altered responsiveness to a specific substance acquired through immunization or natural infection. For certain diseases (e.g., measles, chicken- pox), this protection generally lasts for the life of the individual.

IMMUNIZATION (Syn: vaccination) The artificial induction of active immunity by intro- ducing into a vulnerable host the specific antigen of a pathogenic organism. Protection of susceptible individuals from communicable disease by administration of a living modi- fied agent (as in yellow fever), a suspension of killed organisms (as in whooping cough), or an inactivated toxin (as in tetanus). Temporary passive immunization can be produced by administration of antibody in the form of immune globulin in some conditions.

IMMUNIZATION, LATENT The process of developing immunity by a single or repeated inapparent asymptomatic infection. Not necessarily related to latent infection. See also immunity, acquired.

IMMUNOGENICITY The ability of an infectious agent to induce specific immunity.

IMPACT FACTOR See bibliographic impact factor (BIF).

IMPACT FRACTION A generalization of population attributable fraction that accommodates both hazardous and protective exposures, multiple levels of exposure, incomplete elimination of exposure, diffusion, or response to exposure. To define this measure, let IF be the impact fraction, let p′ and p′′ be the prevalences of an exposure level before and after an intervention program, and let RR be the risk ratio or rate ratio (properly adjusted for bias) relative to a common reference level, depending on whether impact on risks or rates is being estimated. Then the impact fraction is given by

where the sums are over all exposure categories; in the reference category, RR = 1.

IMPACT NUMBERS Four quantities related to attributable risk, used to communicate risk in terms relevant to four groups (total population, diseased, exposed, and exposed with disease). Expands the concept of number needed to treat. (1) Population impact number (PIN) is the number of subjects in the total population among whom one case is attributable to the risk factor. (2) Case impact number (CIN) is the number of people with the disease or outcome among whom one case is attributable to the risk factor. (3) Exposure impact number (EIN) is the number of people with the exposure among whom one case is attributable to the risk factor. (4) Exposed cases impact number (ECIN) is the number of exposed people with the disease or outcome among whom one case is attributable to the risk factor.242

IMPAIRMENT A physical or mental defect at the level of a body system or organ. See also international classification of impairments, disabilities, and handicaps for the official WHO definition.


  1. A form of learning behavior (e.g., the expression of affection) in a critical period of development early in a young animal’s life, including the life of a human; it may result from the interaction between mother and infant. The imposition of a stable behavior pattern in a young animal by exposure, during a particular period in its development, to one of a restricted set of stimuli.243
  2. An epigenetic phenomenon in which the expression of a gene varies depending on whether it is inherited from the mother or the father.

INAPPARENT INFECTION (Syn: subclinical infection) The presence of infection in a host without occurrence of recognizable clinical signs or symptoms. Of epidemiologic significance because hosts so infected, although apparently well, may serve as silent or inapparent disseminators of the infectious agent. See also disease, preclinical; dis- ease, subclinical; vector-borne infection.

INCEPTION COHORT A group of individuals identified and assembled for subsequent study at an early and uniform point in the course of the specified health condition; e.g., near the onset (inception) of symptoms, soon after diagnosis, at detection of a clinically significant pathological event. Thus, subjects who succumbed to or completely recov- ered from the disorder are included with those whose disease persisted.62,63,81 Failure to select an inception cohort often severely biases studies on the natural history of disease. See also study base.

INCEPTION RATE The rate at which new spells of illness occur in a population. A term applied principally to short-term spells of illness, such as acute respiratory infections, and preferred by some epidemiologists because an annual incidence rate for such con- ditions may exceed the numbers in the population at risk.

INCIDENCE The number of instances of illness commencing, or of persons falling ill, during a given period in a specified population. More generally, the number of new health-related events in a defined population within a specified period of time. It may be measured as a frequency count, a rate, or a proportion.

INCIDENCE DENSITY The average person-time incidence rate. Sometimes used to describe the hazard rate. See force of morbidity; incidence rate.

INCIDENCE-DENSITY RATIO (IDR) The ratio of two incidence densities. See also rate ratio. INCIDENCE PROPORTION (Syn: cumulative incidence) Incidence expressed as a pro- portion of the population at risk. A measure of risk. The time duration must be specified for it to be meaningful. It approximates to the incidence rate multiplied by the time duration when that product is small. The proportion of a closed population at risk for a disease that develops the disease during a specified interval.

INCIDENCE RATE The Rate at which new events occur in a population.The numerator is the number of new events that occur in a defined period or other physical span. The denominator is the population at risk of experiencing the event during this period, sometimes expressed as person-time; it may instead be in other units, such as passenger-miles. The incidence rate most often used in public health practice is calculated from the formula

Strictly speaking, this ratio is neither a rate nor a proportion but is instead the rate multiplied by the length of the specified period. If the period is a year, the ratio is nonetheless often called the annual incidence rate. The average size of the population is often the estimated population size at the midperiod. The ratio divided by the length of the period is an estimate of the person-time incidence rate (i.e., the rate per 10 n person- years). If the ratio is small, as with many chronic diseases, it is also a good estimate of the cumulative incidence over the period (e.g., a year). If the number of new cases during a specified period is divided by the sum of the person-time units at risk for all persons during the period, the result is the person-time incidence rate.

INCIDENCE RATE RATIO The incidence rate in the exposed group divided by the incidence rate in the unexposed group. Often referred to as the rate ratio.

INCIDENCE STUDY See cohort study.

INCIDENT NUMBER See incidence.

INCLEN Acronym for International Clinical Epidemiology Network, which consists of clinical epidemiology units (CEUs) in about 30 countries, mostly developing nations. Each CEU includes clinical epidemiologists, statisticians, economists, and social scien- tists. The network began at the initiative of Kerr L. White (b. 1916), with financial sup- port from the Rockefeller Foundation.


  1. The time interval between invasion by an infectious agent and appearance of the first sign or symptom of the disease in question. See also latent period.
  2. In a vector, the period between entry of the infectious agent into the vector and the time at which the vector becomes infective; i.e., transmission of the infectious agent from the vector to a fresh final host is possible (extrinsic incubation period).

INDEPENDENCE, STATISTICAL Two events are said to be (statistically) independent if the occurrence of one is in no way predictable from the occurrence of the other.

× 10n

Two variables are said to be independent if the distribution of values of one is the same for all values of the other. Independence is the antonym of association.


  1. The characteristic being observed or measured that is hypothesized to influence an event or manifestation (the dependent variable) within the defined area of relationships under study; that is, the independent variable is not influenced by the event or manifestation but may cause or contribute to variation of the event or manifestation.
  2. In regression analysis, an independent variable is one of (perhaps) several variables that appear as arguments in a regression equation; a regressor or covariate.20,31 See also dependent variable; regressand; regressor.


  1. In epidemiology and related sciences, this word usually means a rating scale, e.g., a set of numbers derived from a series of observations of specified variables. Examples include the many varieties of health status index, scoring systems for severity or stage of cancer, heart murmurs, mental retardation, etc.
  2. One way to present a measurement with adjustment to the results of other measurements; e.g., the Quetelet Index II, now called body-mass index (BMI), is an index of this type (weight is corrected for height).

INDEX CASE The first case in a family or other defined group to come to the attention of the investigator. See also propositus.

INDEX GROUP (Syn: index series)

  1. In an experiment, the group receiving the experimental regimen
  2. In a case-control study, the cases
  3. In a cohort study, the exposed group

INDICATOR VARIABLE In statistics, a variable taking only one of two possible values, one (usually 1) indicating the presence of a condition, and the other (usually zero) indi- cating absence of the condition. Used mainly in regression analysis.

INDIRECT ADJUSTMENT See standardization.

INDIRECT COSTS See cost, indirect.

INDIRECT OBSTETRIC DEATH See maternal mortality.

INDIVIDUAL THINKING In medicine, it is the ability to make the best prediction in terms of diagnosis and prognosis for the individual patient and to adapt the management and treatment to the unique characteristics of an essentially unpredictable person. Medicine is the art of individual thinking.10,61–63 See also atomistic fallacy; clinical study; ecological fallacy; population thinking; prevention paradox; strategy, “high-risk.”

INDIVIDUAL VARIATION Two types are distinguished:

  1. Intraindividual variation: The variation of biological variables within the same
    individual, depending upon circumstances such as the phase of certain body rhythms and the presence or absence of emotional stress.These variables do not have a precise value but rather a range. Examples include diurnal variation in body temperature, fluctuation of blood pressure, blood sugar, etc.
  2. Interindividual variation: As used by Darwin, the term means variation between individuals. This is the preferred usage; the first usage is better described as personal variation.


  1. Any method of rational or logical analysis that proceeds from the particular to the general. Although no infallible method of logical reasoning exists, general theories require induction. Conceptually bright ideas and breakthroughs and ordinary statistical inference belong to the realm of induction. Contrast deduction. See also hypothetico-deductive method; inductive logic; inference.
  2. Causation of the initiating events of a disease process (e.g., induction of initiating mutations at the beginning of carcinogenesis).

INDUCTION PERIOD The interval between initiation of exposure to the causal agent and initiation of the health process; e.g., from onset of exposure to the disease- causing agent to initiation of the disease. See also carcinogen; incubation period; latency period.

INDUCTIVE LOGIC Logic that seeks to reach generalizations by reasoning from an assembly of particular observations. Francis Bacon was its first proponent as applied to science. It remains an important mode of (and a collection of methods for) scientific reasoning. Its status as a “logic” has long been debated, and many authors prefer to use the term induction instead. Scholars have argued that statistical theories are attempts to put induction on a logically sound footing.6–12 Nonetheless, epidemiology challenges these attempts insofar as epidemiologic reasoning must go beyond statistics. See also hypothetico-deductive method; inference.

INDUSTRIAL HYGIENE The science devoted to recognition, evaluation, and control of those environmental factors or stresses arising from or in the workplace that may cause sickness, impaired health and well-being, or significant discomfort and inefficiency among workers or among persons in the community. Alternatively, the profession that anticipates and controls unhealthy conditions of work to prevent illness among employ- ees. See also occupational health.

INEQUALITIES IN HEALTH The virtually universal phenomenon of variation in health indicators (e.g., infant and maternal mortality rates, mortality and incidence rates of many diseases) in association with socioeconomic status and ethnicity. It has been observed since the vital statistics of England and Wales were examined by William Farr (1807–1883) and reported annually from 1840. The gap between best and worst health experience has widened in recent decades in many rich nations.

INFANT MORTALITY RATE (IMR) A measure of the yearly rate of deaths in children less than 1 year old. The denominator is the number of live births in the same year. Defined as

INFECTIBILITY The host characteristic or state in which the host is capable of being infected. See also infectiousness; infectivity.

INFECTION (Syn: colonization) The entry and development or multiplication of an infectious agent in the body of man or animals. Infection is not synonymous with infectious disease; the result may be inapparent or manifest. The presence of living infectious agents on exterior surfaces of the body is called “infestation” (e.g., pedic- ulosis, scabies). The presence of living infectious agents upon articles of apparel or soiled articles is not infection, but represents contamination of such articles. See also inapparent infection; transmission of infection.

INFECTION, GRADIENT OF The range of manifestations of illness in the host reflecting the response to an infectious agent, which extends from death at one extreme to inapparent infection at the other. The frequency of these manifestations varies with the specific infectious disease. For example, human infection with the virus of rabies is almost invariably fatal, whereas a high proportion of persons infected in childhood with the virus of hepatitis A experience a subclinical or mild clinical infection.

INFECTION, LATENT PERIOD OF The time between initiation of infection and first shedding or excretion of the agent.

INFECTION, SUBCLINICAL See inapparent infection.

INFECTION RATE The incidence rate of manifest plus inapparent infections (the latter determined by seroepidemiology).

INFECTION TRANSMISSION PARAMETER (r) The proportion of total possible contacts between infectious cases and susceptibles that lead to new infections.

INFECTIOUS DISEASE See communicable disease.

INFECTIOUSNESS A characteristic of a disease that concerns the relative ease with which it is transmitted to other hosts. A droplet spread disease, for instance, is more infectious than one spread by direct contact. The characteristics of the portals of exit and entry are thus also determinants of infectiousness, as are the agent characteristics of ability to survive away from the host and of infectivity.


1. The characteristic of the disease agent that embodies capability to enter, survive, and multiply in the host. A measure of infectivity is the secondary attack rate.

2. The proportion of exposures, in defined circumstances, that results in infection.

INFERENCE The process of passing from observations and axioms to generalizations. In statistics, the development of generalization from sample data, usually with calculated degrees of uncertainty. Causal inference from observational data is a key task of epidemiology and other sciences as sociology, education, behavioral sciences, demography, economics, or health services research; these disciplines share methodological frameworks for causal inference.

INFESTATION The development on (rather than in) the body of a pathogenic agent;

e.g., body lice. Some authors use the term also to describe invasion of the gut by parasitic worms.

INFLUENCE ANALYSIS Methods to determine the robustness of an assessment by examining the extent to which results are affected by changes in recorded measurements and variables. The aim is to identify recorded values that made the largest contribution to the results or to find a solution that is relatively stable for the most commonly occurring values of these variables. See also outliers; sensitivity analysis.

INFORMATICS The study of information and the ways to handle it, especially by means of information technology, i.e., computers and other electronic devices for rapid trans- fer, processing, and analysis of large amounts of data.

INFORMATION Facts that have been arranged and/or transformed to provide the basis for interpretation and conversion into knowledge.

INFORMATION BIAS (Syn: observational bias)

  1. A flaw in measuring exposure, covariate, or outcome variables that results in
    different quality (accuracy) of information between comparison groups.10,12,14,31,95 The occurrence of information biases may not be independent of the occurrence of selection biases.
  2. Bias in an estimate arising from measurement errors.

INFORMATION SYSTEM As applied in epidemology, a combination of vital and health statistical data from multiple sources, used to derive information about the health needs, health resources, costs, use of health services, and outcomes of use by the population of a specified jurisdiction. The term may also describe the automatic release from computers of stored information in response to programmed stimuli. For example, parents can be notified when their children are due to receive booster doses of an immunizing agent against infectious disease.

INFORMATION THEORY Mathematical theory dealing with the nature, effectiveness, and accuracy of information transfer.

INFORMED CONSENT Voluntary consent given by a subject or a responsible proxy (e.g., a parent) for participation in a study, immunization program, treatment regimen, etc., after being informed of the purpose, methods, procedures, potential benefits and potential harms, and, when relevant, the degree of uncertainty about such outcomes. The essential criteria of informed consent are that the subject has both knowledge and comprehension, that consent is freely given without duress or undue influence, and that the right of withdrawal at any time is clearly communicated to the subject. Other aspects of informed consent in the context of epidemiological and biomedical research, and criteria to be met in obtaining it, are specified in International Ethical Guidelines for Epidemiological Studies (Geneva: CIOMS/WHO, 2008) and International Ethical Guidelines for Biomedical Research Involving Human Subjects (Geneva: CIOMS/ WHO, 2002).244 See also ethics.

INGELFINGER RULE Rule developed by Franz Ingelfinger (1910–1980), former editor of the New England Journal of Medicine, as follows: “The Journal undertakes review with the understanding that neither the substance of the article nor the figures or tables have been published or will be submitted for publication during the period of review. This restriction does not apply to abstracts published in connection with scientific meet- ings or to news reports based on public presentations at such meetings.”245

A revision of the rule imposed a news embargo246 until the pertinent article is published. The Ingelfinger rule (or modifications of it) has been adopted by many high-quality peer-reviewed biomedical and science journals. The aims of the rule are to eliminate duplicate publication and reduce uncritical acceptance of original work prior to peer review and publication.247

INHERITANCE In the biological sciences, pattern followed by the transmission from gen- eration to generation of a given phenotype (e.g., a disease). There are several types of complex inheritance (non-Mendelian) and of Mendelian inheritance (dominant, reces- sive, sex-linked).23 See also epigenetic inheritance; heredity.

INHERITANCE, CULTURAL The transfer from one generation to another of values, beliefs, and customs (e.g., solidarity values, patriotic feelings, leisure habits). Process through which individuals receive, learn, and adopt norms and behaviors (e.g., on smoking and alcohol use, exercise, sleeping and dietary patterns, symptom reporting, meaning-making) from the family, in school, or in other spheres of society.248–251

INJURY The transfer of one of the forms of physical energy (mechanical, chemical, thermal) in amounts or at rates that exceed the threshold of human tolerance. It may also result from lack of essential energy such as oxygen (e.g., drowning) or heat (e.g., hypothermia).252

INOCULATION See vaccination. INPUT

  1. The sum total of resources and energies purposefully engaged (e.g., in a health program) in order to intervene in the spontaneous operation of a system.
  2. The basic resources required in terms of manpower, money, materials, and time.

INSERM Institut National de la Santé et de la Recherche Médicale (France). The French national institute for medical and health research.


INSTITUTIONAL REVIEW BOARD (IRB) The term used in the United States to describe the standing committee in a medical school, hospital, or other health care facility that is charged with ensuring the safety and well-being of human subjects involved in research. The IRB is responsible for ethical review of research proposals. Many synonyms are used in other countries, e.g., Ethical Review Committee, Research Ethics Board. All research, including epidemiologic research, that involves human subjects must be approved by an institutional review board or equivalent body.

INSTRUMENTAL ERROR Error due to faults arising in any or in all aspects of a measuring instrument, i.e., calibration, accuracy, precision, etc. Also applied to error arising from impure reagents, wrong dilutions, etc.


  1. A device or questionnaire used to obtain measurements
  2. An instrumental variable

INSTRUMENTAL VARIABLE See instrumental variable analysis.

INSTRUMENTAL VARIABLE ANALYSIS Method originally used in econometrics and some social sciences that, under certain assumptions, allows the estimation of causal effects even in the presence of unmeasured confounding for the exposure and effect of interest. An instrumental variable, or instrument, has to meet the following conditions: (1) it is associated with the exposure, (2) it affects the outcome only through the expo- sure, and (3) it does not share any (uncontrolled) common cause with the outcome.253 See also confounding bias; residual confounding.


  1. The action or process of integrating.To integrate: to make a new whole; to combine parts into a new system and get them to interact so that the system expresses functions unavailable to the parts. The organizing of elements to form a coherent whole or system. Integration of knowledge from different scientific disciplines yields knowledge that no discipline alone achieves.
  2. In health promotion and disease prevention, strategies that target several risk factors, use multiple strategies at various levels of influence, and require intersectoral action. Integration entails multiplicity (more than one risk factor, level, sector, agent), and synergy resulting from multiplicity.
  3. In mathematics, the process of finding an antiderivative of a given function, or its value over a specified interval.
  4. In neurophysiology, the blending together of the excitatory and inhibitory nerve impulses that arrive through the thousands of synapses at a nerve cell body.
  1. In medicine, the coordination within the brain of different nervous processes (e.g., sensory information from the inner ear and the eye must be integrated by the brain with other stimuli for the sense of balance and to control posture). (This definition can be a metaphor for integrative research in many scientific fields).
  2. In economics, the merging and fusion of companies or national economies.

Integration is no less crucial to science than to the functioning of postmodern societies. Examples: quality public transportation favors integration of disabled individuals and disadvantaged groups into society; integration of racial and ethnic minorities into the educational system; integration of preventive services into clinical care; political and economic processes favor integration into the European Union of Eastern European states. Synonyms, analogies and metaphors are here useful as well: integration involves and refers to interaction, dialogue, complicity, performance, symbiosis, sharing, pooling, porousness, amalgamation, merging, coalescing, fusing, welding, blending, weaving …

INTEGRATIVE RESEARCH Research that integrates knowledge, data, methods, techniques, reasoning, and other scientific and cultural referents from multiple disciplines, approaches, and levels of analysis to generate knowledge that no discipline alone could achieve. For instance, research that integrates cultural, economic, and other “macro- level” or contextual factors with individual factors, as in mutilevel analysis; analyses of the relationships among gene structure, expression, and function; research on the relationships among molecular pathways, pathophysiology, and clinical phenotypes, as in clinical pharmacology and clinical genetics; research that integrates interactions among environmental, genetic, and epigenetic processes.173,254 Epidemiology is an inherently integrative discipline, and so are many of its branches, subspecialities, and approaches, like clinical and molecular epidemiology, social epidemiology or environmental epidemiology; developmental and life course epidemiology, for instance, attempts to integrate biological and social risk processes.16 See also clinical study; public health impact assessment; transdisciplinarity; reductionism.

INTENTION-TO-TREAT ANALYSIS A fundamental way to analyze subjects in a ran- domized controlled clinical trial255,256: all patients allocated to each arm of the treat- ment regimen are analyzed together “as intended” upon randomization, whether or not they actually received or completed the prescribed regimen. Failure to follow this step defeats the main purpose and advantage of random allocation and can cause serious bias. This approach is virtually always required in studies aiming to influence clinical or public health practice. It may be complemented by an explanatory analysis, in which some participants (e.g., poor compliers) are excluded from analyses.14 Because of its pragmatic approach, intention-to-treat analysis can underestimate drug efficacy; instrumental variable analysis and g-estimation can be used to address this bias while making use of the random allocation. See also effectiveness; pragmatic study.


  1. The interdependent operation of two or more causes to produce, prevent, or control an effect. Biological interaction means the interdependent operation of two or more biological causes to produce, prevent, or control an effect. See also antagonism; synergism.
  2. Differences in the effect measure for one factor at different levels of another factor. See also effect modification; effect modifier.
  3. The necessity for a product term in a linear model (Syn: statistical interaction).

131 International classification of primary care

INTERMEDIATE VARIABLE [Syn: contingent variable, intervening (causal) variable, mediator variable] A variable that occurs in a causal pathway from a causal (indepen- dent) variable to an outcome (dependent) variable. It causes variation in the outcome variable and itself is caused to vary by the original causal variable. Such a variable will be associated with both the causal and the outcome variables.

INTERNAL VALIDITY See validity, study.

INTERNATIONAL CLASSIFICATION OF DISEASES (ICD) The classification of specific conditions and groups of conditions determined by an internationally representative group of experts who advise the World Health Organization, which publishes the complete list in periodic revisions. Every disease entity is assigned a number. There are 21 major divisions (chapters) and a hierarchical arrangement of subdivisions (rubrics) within each in the tenth revision. Some chapters are “etiological,” e.g., Infective and Parasitic Conditions; others relate to body systems, e.g., Circulatory System; and some to classes of condition, e.g., neoplasms, injury (violence). The heterogeneity of catego- ries reflects prevailing uncertainties about causes of disease (and classification in rela- tion to causes). The tenth revision of the manual (ICD-10) was published by WHO in 1990, after ratification in 1989. See also international statistical classification of diseases and related health problems (ICD-10).

INTERNATIONAL CLASSIFICATION OF HEALTH PROBLEMS IN PRIMARY CARE (ICHPPC) See International Classification of Primary Care, Second Edition Revised (ICPC-2-R).

INTERNATIONAL CLASSIFICATION OF IMPAIRMENTS, DISABILITIES, AND HAND- ICAPS (ICIDH) First published by WHO in 1980, this is an attempt to produce a sys- tematic taxonomy of the consequences of injury and disease.

An impairment is defined in ICIDH as any loss or abnormality of psychological, physiological, or anatomical structure or function. It is concerned with abnormalities of body structure and appearance and with organ or system function resulting from any cause; in principle, impairments represent disturbances at the organ level.

A disability is defined in ICIDH as any restriction or lack (resulting from an impairment) of ability to perform an activity in a manner or within the range considered normal for a human being. The term disability reflects the consequences of impairment in terms of functional performance and activity by the individual; disabilities thus represent disturbances at the level of the person.

A handicap is defined in ICIDH as a disadvantage for a given individual, resulting from an impairment or a disability, that limits or prevents the fulfillment of a role that is normal (depending on age, sex, and social and cultural practice) for that individual. The term handicap thus reflects interaction with and adaptation to the individual’s surroundings.22 ICIDH has been superseded by the International Classification of Functioning, Disability and Health.

INTERNATIONAL CLASSIFICATION OF PRIMARY CARE, SECOND EDITION REVISED (ICPC-2-R) The official classification of the World Organization of Family Doctors (WONCA).257,258 WONCA is also known as the World Organization of Family Doctors. ICPC includes three elements of the doctor-patient encounter: the reason for encounter (RFE), the diagnosis, and the treatment or other action or intervention. It is a biaxial classification system based on chapters and components. It uses three-digit alphanumeric codes with mnemonic qualities to facilitate its day-to-day use. Chapters, each with an alpha code, form one axis; components with rubrics having a two-digit numeric code form the second axis. The components deal with symptoms and com- plaints, diagnoses and therapeutic interventions, administrative procedures and dis- eases. ICPC includes a detailed conversion system for linking with ICD-10 codes. See also problem-oriented medical record.

INTERNATIONAL COMPARISONS In epidemiology and public health, comparing regions or nations of the world in terms of disease determinants or outcomes, as in tables that show the rank order of vital statistics such as infant mortality rates, death or incidence rates for cancer, heart disease, etc. The dangers of making comparisons include the shifting tides of diagnostic fashion and the varying criteria and definitions that prevail from one nation to another. Only after ensuring that like is truly being com- pared with like can the comparisons be trusted, and even then only with reservations about validity. See also cross-cultural study.

INTERNATIONAL FORM OF MEDICAL CERTIFICATE OF CAUSES OF DEATH In adopting the tenth revision of ICD in 1990, the World Health Assembly resolved that causes of death to be entered on the medical certificate of cause of death are all those dis- eases, morbid conditions, or injuries that either resulted in or contributed to death and the circumstances of the accident or violence that produced such injuries. Antecedent causes and other significant conditions are also to be recorded. See death certificate.

INTERNATIONAL NOMENCLATURE OF DISEASES (IND) Since 1970, the Council for International Organizations of the Medical Sciences (CIOMS) and the WHO have collaborated in preparing an International Nomenclature of Diseases (IND). This is a complement to the ICD. The purpose of the IND is to provide a single recommended name for every disease entity. The criteria for selection are that the name should be specific, unambiguous, as self-descriptive and as simple as possible, and based on cause whenever feasible. A list of synonyms is appended to each definition.

INTERNATIONAL STATISTICAL CLASSIFICATION OF DISEASES AND RELATED HEALTH PROBLEMS The tenth revision, known in short as ICD-10, was approved by the International Conference for the Tenth Revision in 1989 and by the 43rd World Health Assembly in 1990. It is the latest in a series of international classifications dating back to the Bertillon classification (i.e., the International List of Causes of Death, 1893); ICD-10 came into effect at the beginning of 1993, exactly 100 years after the original. The tenth revision has 21 chapters and uses an alphanumeric coding system in order to provide a larger coding frame than previously, leaving room for future expan- sion. The chapters of ICD-10 are as follows:

  • I  (A00–B99): Certain infectious and parasitic diseases
  • II  (C00–C97): Neoplasms
  • III  (D50–D89): Diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism
  • IV  (E00–E90): Endocrine, nutritional, and metabolic diseases
  • V  (F00–F99): Mental and behavioral disorders
  • VI  (G00–G99): Diseases of the nervous system
  • VII  (H00–H59): Diseases of the eye and adnexa
  • VIII  (H60–H95): Diseases of the ear and mastoid process
  • IX  (I00–I99): Diseases of the circulatory system
  • X  (J00–J99): Diseases of the respiratory system
  • XI  (K00–K93): Diseases of the digestive system
  • XII (L00–L99): Diseases of the skin and subcutaneous tissue
  • XIII  (M00–M99): Diseases of the musculoskeletal system and connective tissue
  • XIV  (N00–N99): Diseases of the genitourinary system
  • XV  (000–099): Pregnancy, childbirth, and the puerperium
  • XVI  (P00–P96): Certain conditions originating in the perinatal period
  • XVII  (Q00–Q99): Congenital malformations, deformations, and chromosomal abnormalities
  • XVIII  (R00–R99): Symptoms, signs, and abnormal clinical and laboratory findings not elsewhere classified
  • XIX  (S00–T98): Injury, poisoning, and certain other consequences of external causes
  • XX  (V01–Y99): External causes of morbidity and mortality
  • XXI  (Z00–Z99): Factors influencing health status and contact with health services

INTERPOLATE, INTERPOLATION To predict the value of variates within the range of observations; the resulting prediction.

INTERPRETIVE BIAS (Syn: interpretative bias, bias of interpretation)

  1. Error arising from inference and speculation. Among others, two sources of the error are (a) failure of the investigators to consider all relevant and rationally defensible interpretations coherent with the facts and to assess the scientific support of each interpretation and (b) mishandling of cases that constitute exceptions to some general conclusion.
  2. Errors that can occur in interpretative and evaluative processes of scientific evidence.34

The interpretive process is a necessary and inevitable aspect of science. Science commonly has subjective and cultural components.259 Although unbiased interpretation of data is as important as performing rigorous experiments, evaluative processes are seldom totally objective or completely independent of scientists’ convictions and theoretical apparatus. Good science inevitably embodies a tension between concrete data and deeply held convictions. Subjectivity is always alive at the cutting edge of scientific progress, where new ideas develop.260 Lack of objective measures for the subjective components of interpretation makes no less important the interaction between data and judgment. Recognition of interpretative processes in epidemiology should not lead to a naive relativism or to deem all claims to knowledge equally valid because of subjectivity. A view that science is totally objective is mythical and ignores the human element of epidemiological inquiry.34,127 See also auxiliary hypothesis bias; cognitive dissonance bias; confirmation bias; mechanistic bias; publication bias; rescue bias; “time will tell” bias.

INTERSECTORAL ACTION (Syn: intersectoral collaboration) Activities and strategies involving several components of the body politic (e.g., the health sector, the education sector, the housing sector) that, working together, can enhance health conditions more effectively than when working independently of one another. See also health in all policies; healthy public policies.

INTERVAL The set containing all numbers between two given numbers.

INTERVAL ESTIMATE An interval within which a parameter under study (such as a rel- ative risk) is stated to lie with a particular degree of confidence, likelihood, or probabil- ity based on an analysis of a study or multiple studies. See also confidence interval; likelihood interval; posterior interval.

INTERVAL INCIDENCE DENSITY See person-time incidence rate.

INTERVAL SCALE See measurement scale.

INTERVENING CAUSE See intermediate variable.


  1. Synonym for intermediate variable
  2. A variable whose value is altered in order to block or alter the effect(s) of another

See also causality.

INTERVENTION INDEX An estimate of the impact of a therapeutic or preventive inter- vention.261 It is the ratio of the number of persons whose risk level must change to prevent one premature death to the total number at risk. See also absolute risk reduc- tion (ARR); impact numbers; number needed to treat (NNT); relative risk reduc- tion (RRR).

INTERVENTION STUDY An investigation involving intentional change in some aspect of the status of the subjects, e.g., introduction of a preventive or therapeutic regimen or an intervention designed to test a hypothesized relationship; usually an experiment such as a randomized controlled trial.

INTERVIEWER BIAS Systematic error due to interviewers’ subconscious or conscious gathering of selective data or influencing subject response.

INTERVIEW SCHEDULE The precisely designed set of questions used in an interview. See also survey instrument.


  1. In sibship studies in genetics, the variance of genotypes as a proportion of the variance of phenotypes.
  2. In surveys and group-randomized studies, the extent to which members of a group (cluster) resemble each other more than they resemble members of other groups (clusters). See design effect.

INVERSE PROBABILITY WEIGHTING A method for the estimation of causal effects under the assumption of no unmeasured confounders. An extension of inverse weight- ing methods used in survey sampling and missing data analysis. Individuals in the study population are weighted by the inverse of their probability of having the level of exposure they actually have given their covariates, creating a pseudopopulation in which, theoret- ically, there is no confounding by the covariates used to estimate the weights.262 See also G-estimation; marginal structural models.

INVOLUNTARY SMOKING (Syn: passive smoking) Exposure to secondhand tobacco smoke, a mixture of exhaled mainstream smoke and sidestream smoke released from a smoldering cigarette or other smoking device (e.g., cigar, pipe, bidi) and diluted with ambient air. It involves inhaling carcinogens and other toxic components present in secondhand tobacco smoke; the latter is sometimes referred to as “environmental” tobacco smoke (ets). It includes both smoke exhaled by smokers and smoke released directly from burning tobacco into ambient air; the latter is called sidestream smoke and contains higher proportions of carcinogenic substances and other toxic agents than exhaled smoke. The adjective involuntary is preferable to passive, as the latter may imply acquiescence. Involuntary smoking (exposure to secondhand or “environmental” tobacco smoke) is carcinogenic to humans (group 1 of IARC).

ISLAND POPULATION A group of individuals isolated from larger groups and possess- ing a relatively limited gene pool; alternatively, a group that is immunologically isolated and may therefore be unduly susceptible to infection with alien pathogens.

ISEE International Society for Environmental Epidemiology (

ISODEMOGRAPHIC MAP (Syn: density-equalizing map) A diagrammatic method of displaying administrative jurisdictions of a country in two-dimensional “maps” with areas directly proportional to the population of the jurisdictions. Thus densely popu- lated urban regions occupy large areas of the map and sparsely inhabited rural regions occupy small areas. Additional data—such as incidence or mortality rates within each jurisdiction—can be superimposed in colors or shading to represent rates.

ISOLATE (noun)

  1. In genetics, a subpopulation, generally small, in which matings take place exclusively
    with other members of the same subpopulation
  2. In microbiology, a pure culture of an organism


  1. In microbiology, the separation of an organism from others, usually by making serial cultures.
  2. Separation, for the period of communicability, of infected persons or animals from others under such conditions as to prevent or limit the transmission of the infectious agent from those infected to those who are susceptible or who may spread the agent to others.The CDC Guidelines for Isolation Precautions in Hospitals (1990) expanded on the blood and body fluid precautions described below. Control of Communicable Disease Manual 52 lists seven categories of isolation:

a. Strict isolation: To prevent transmission of highly contagious or virulent infections that may be spread by both air and contact. The specifications include a private room and the use of masks, gowns, and gloves for all persons entering the room. Special ventilation requirements with the room at negative pressure to surrounding areas are desirable.

b. Contact isolation: For less highly transmissible or serious infections and diseases or conditions that are spread primarily by close or direct contact. A private room is indicated but patients infected with the same pathogen may share a room. Masks are indicated for those who come close to the patient, gowns are indicated if soiling is likely, and gloves are indicated for touching infectious material.

c. Respiratory isolation: To prevent transmission of infectious diseases over short distances through the air, a private room is indicated, but patients infected with the same organism may share a room. In addition to the basic requirements, masks are indicated for those who come in close contact with the patient; gowns and gloves are not indicated.

d. Tuberculosis isolation (AFB isolation): For patients with pulmonary tuberculosis who have a positive sputum smear or chest x-rays that strongly suggest active tuberculosis. Specifications include use of a private room with special ventilation and closed door. Masks are used only if the patient is coughing and does not reliably and consistently cover the mouth. Gowns are used to prevent gross contamination of clothing. Gloves are not indicated.

e. Enteric precautions: For infections transmitted by direct or indirect contact with feces. Specifications include use of a private room if patient hygiene is poor. Masks are not indicated; gowns should be used if soiling is likely, and gloves are to be used for touching contaminated materials.

f. Drainage/secretion precautions: To prevent infections transmitted by direct or indirect contact with purulent material or drainage from an infected body site.

g. A private room and masking are not indicated; gowns should be used if soiling is likely and gloves used for touching contaminated materials.
Blood/body fluid precautions: To prevent infections that are transmitted by direct or indirect contact with infected blood or body fluids. In addition to the basic requirements, a private room is indicated if patient hygiene is poor; masks are not indicated; gowns should be used if soiling of clothing with blood or body fluids is likely. Gloves should be used for touching blood or body fluids. Blood and body fluid precautions should be used consistently for all patients regardless of their blood-borne infection status (“universal blood and body fluid precautions”). These are intended to prevent parenteral, mucous membrane, and nonintact- skin exposure of health care workers to blood-borne pathogens. Protective barriers include gloves, gowns, masks, and protective eyewear. See also universal precautions. See also quarantine.

ISOMETRIC CHART A chart or graph that portrays three dimensions on a plane surface.

IVDU Intravenous drug user, a high-risk group for transmission of blood-borne infection, especially HIV and hepatitis C.