1. By historical and common usage, the group (subspecies in traditional scientific use) or a person who belongs to as a result of a mix of physical features such as skin color and hair texture, which reflect ancestry and geographical origins; as identified by others; or, increasingly, as self-identified. The importance of social factors in the creation and perpetuation of racial categories has led to broadening of the concept to include a common social and political heritage, making its use similar to ethnicity.188
  1. In biology, a category used in the classification of organisms or a group of individuals within a species that are geographically, ecologically, physiologically, or chromosomally distinct from other members of the species.331

Biological classification of human races is difficult—and sometimes meaningless—because of significant genetic overlaps among population groups. Social scientists have challenged the biological definition of race, arguing that the concept of race most often reflects social and ideological conventions.332 Economic, social, cultural, and behavioral differences are more important than biological differences in determining health status. However, race is a useful concept from the public health perspective because some diseases are strongly correlated with biological aspects of race; this may relate to gene- environment interaction or to the presence of specific genes, which may be due to environmental exposures of prior generations. Useful insights into human biology and genetics derive from analysis by racial group of large data sets such as the census and national health surveys. See also ethnic group.

RADIX The size of the hypothetical birth cohort in a life table, commonly 1000 or 100,000.

RAHE-HOLMES SOCIAL READJUSTMENT RATING SCALE See life events. RANDOM (Syn: aleatory, stochastic) Governed by chance; not completely determined by measurable factors.

RANDOM ALLOCATION, RANDOMIZATION Allocation of individuals to groups in a clinical trial (e.g., intervention and control) by chance. It makes the trial a randomized controlled trial. It makes differences between the intervention and control groups random. Within the limits of chance variation (e.g., if the number of subjects is large), it yields groups similar at the start of an investigation and does so for both known and unknown variables (i.e., including measured and unmeasured determinants of the outcomes). No other methodological procedure can accomplish this. Randomization enables statistical procedures to account for uncertainty about unmeasured differences via standard errors, P values, and confidence intervals. It also ensures that personal judgment and views of the investigator do not influence allocation (e.g., of treatment). Random allocation should not be confused with haphazard assignment: random allocation follows a predetermined plan that is usually devised with the aid of a computer program. Unsatisfactory (nonrandom) methods are allocation by alternation or date of birth, case record, day of the week, presenting or enrollment order. These methods, sometimes called “pseudorandomization,” are not reliable in producing similar groups, prone to breakdown of allocation concealment, and not accepted as appropriate allocation methods. See also blocked randomization; confounding bias; stratified randomization.

RANDOM-DIGIT DIALING A method for sampling people in telephone surveys in which telephone numbers are randomly dialed.

RANDOMIZATION, MENDELIAN See Mendelian randomization.

RANDOMIZED CONTROLLED TRIAL (RCT) An epidemiological experiment in which subjects in a population are randomly allocated into groups, usually called study and control groups, to receive or not to receive an experimental preventive or therapeutic procedure, maneuver, or intervention. The results are assessed by rigorous comparison of rates of disease, death, recovery, or other appropriate outcome in the study and control groups. RCTs are generally regarded as the most scientifically rigorous method of hypothesis testing available in epidemiology and medicine. Nonetheless, they may suffer serious lack of generalizability, due, for example, to the nonrepresentativeness of patients who are ethically and practically eligible, chosen, or consent to participate. A few authors refer to this method as “randomized control trial.” See also community trial; clinical trial; experimental epidemiology.

RANDOM SAMPLE A sample that is arrived at by selecting sample units such that each possible unit has a fixed and known or equal probability of selection. See also sample.


1. A variable whose distribution incorporates some element of chance, randomness, or unpredictability.

2. A variable that has or may be assigned a (possibly unknown) probability distribution.

RANDOM WALK The path traversed by a particle that moves in steps, each step being determined by chance in regard to direction, magnitude, or both. The theory of random walks has many applications (e.g., to sequential sampling and to the migration of insects, including disease vectors).

RANGE OF DISTRIBUTION The difference between the largest and smallest values in a distribution.

RANK (v.) To arrange in a meaningful order or sequence (e.g., numerical order, degree of severity).

RANKING SCALE (Syn: ordinal scale) A scale that arrays the members of a group from high to low according to the magnitude of the observations, assigns numbers to the ranks, and neglects distances between members of the array. See also measurement scale.

RAPID EPIDEMIOLOGICAL ASSESSMENT Methods that can be used to yield results as rapidly and efficiently as available resources permit; e.g., to assess health problems and evaluate health programs in developing countries or to delineate the health impact of a public health emergency, such as a disaster or an epidemic with unusual features.333 See also disaster epidemiology; triage.

RARE-DISEASE ASSUMPTION (Syn: rarity assumption) Reliance on the use of approx- imations, based on the assumption that the disease being studied is rare in the studied population. This assumption must be met for (1) prevalence to be approximately equal to the incidence rate multiplied by the average duration of disease (i.e., for the valid- ity of the approximation P = I × D); (2) the incidence proportion to be approximately equal to the incidence rate multiplied by the length of the follow-up period (i.e., for IP = IR × T); and (3) for the odds ratio to be approximately equal to the incidence rate ratio or the risk ratio or cumulative incidence ratio (i.e., OR ≈ IRR or OR ≈ RR) in some but not other case-control studies, depending on the method used to select con- trols. When the density sampling method is used to select controls, OR = IRR regardless of the rarity or frequency of the disease. Decisions about “rarity” are rather arbitrary; the odds ratio will usually be within p% of the risk ratio if the risk does not exceed p% in any group being compared (e.g., if the risk is always below 5%, the odds ratio will generally be within 5% of the risk ratio and even closer to the rate ratio).12

RATE A measure of the frequency of occurrence of a phenomenon. In epidemiology, demography, and vital statistics, a rate is an expression of the frequency with which an event occurs in a defined population, usually in a specified period of time. Physical units other than time may be used for constructing rates, however; for example, in accident epidemiology, deaths per passenger-mile is a more meaningful way of comparing modes of transportation. The use of rates rather than raw numbers is essential for comparison of experience between populations at different times, different places, or among different classes of persons.

The components of a rate are the numerator, the denominator, the specified time in which events occur, and usually a multiplier, a power of 10, that converts the rate from an awkward fraction or decimal to a whole number.

In vital statistics,

In epidemiology, the denominator is usually person-time.

All rates are ratios, calculated by dividing a numerator (e.g., the number of deaths or newly occurring cases of a disease in a given period) by a denominator (e.g., the average population during that period). Some rates are proportions, where the numerator is contained within the denominator. Rate has several different usages in epidemiology: 1. As a wrong synonym for ratio, it refers to proportions as rates, as in the terms

cumulative incidence rate or survival rate. Proportion and ratio are not synonyms for rate.

  1. In other situations, rate refers only to ratios representing relative changes (actual or
    potential) in two quantities. This accords with the Oxford English Dictionary, which
    gives “relative amount of variation” among its definitions for rate.
  1. Sometimes rate is further restricted to refer only to ratios representing changes over time. In this sense, the term prevalence rate is to be avoided, because prevalence cannot (and does not need to) be expressed as a change in time; of course, different prevalence estimates may vary, change, and be compared. In contrast, the force of mortality and the force of morbidity (hazard rate) are proper rates, for they can be expressed as the number of cases developing per unit time divided by the total size
    of the population at risk.

RATE DIFFERENCE (RD) The absolute difference between two rates; for example, the difference in incidence rate between a population group exposed to a causal factor and a population group not exposed to the factor:

where Ie = incidence rate among exposed and Iu = incidence rate among unexposed. In comparisons of exposed and unexposed groups, the term excess rate may be used as a synonym for rate difference.

RATE-ODDS RATIO See odds ratio.

RATE RATIO The ratio of two rates; e.g., the rate in an exposed population divided by the rate in an unexposed population:

where Ie is the incidence rate among the exposed and Iu is the incidence rate among the unexposed. See also relative risk.

RATIO The value obtained by dividing one quantity by another. rate, proportion, and percentage are types of ratios. The numerator of a proportion is included in the population defined by the denominator, whereas in other types of ratios numerator and denominator are distinct quantities, neither being included in the other. The dimensionality of a ratio is obtained through algebraic cancellation, summation, etc., of the dimensionalities of its numerator and denominator terms. Both counted and measured values may be included in the numerator and in the denominator. There are no general restrictions on the dimensionalities or ranges of ratios, but there are in some types of ratios (e.g., proportion, prevalence). Ratios are sometimes expressed as percentages (e.g., standardized mortality ratio). In these cases, the value may exceed 100.

RATIO SCALE See measurement scale.

RAW DATA The entire set of information that has been collected in a study before any cleaning, editing, or statistical manipulation begins.

REASON FOR ENCOUNTER (RFE) The statement of reason(s) why a person enters the health care system, representing that person’s demand for care. The terms recorded by the health care provider clarify the reason for encounter without interpreting it in the form of a diagnosis.258

RECALL BIAS Systematic error due to differences in accuracy or completeness of recall to memory of past events or experiences.12,14,31 For example, a mother whose child has died of leukemia may be more likely than the mother of a healthy living child to remember details of such past experiences as use of x-ray services when the child was in utero.

RECEIVER OPERATING CHARACTERISTIC (ROC) CURVE (Syn: relative operating characteristic curve) A graphic means for assessing the ability of a screening or diagnostic test to discriminate between persons with and without the target disorder. For an ordinal or continuous diagnostic test, the ROC curve depicts the plot of all pairs of sen- sitivity and 1-specificity (false-positive probability) over all possible or chosen cutoff values. The term receiver operating characteristic comes from psychometry, where the characteristic operating response of a receiver-individual to faint stimuli or nonstimuli was recorded. The term was first used in studies of radar during World War II.

RECESSIVE In genetics, a gene that is phenotypically manifest only when present in the homozygous state.23,134,243

RECORD LINKAGE A method for bringing together the information contained in two or more records—e.g., in different sets of medical charts, and in vital records such as birth and death certificates—and a procedure to ensure that each individual is identified and counted only once. This procedure incorporates a unique identifying system such as a personal identification number and/or birth name(s) of the individual’s mother.334

Record linkage makes it possible to relate significant health events that are remote from one another in time and place or to bring together records of different individuals (e.g., members of a family). The resulting information is generally stored and retrieved by a computer, which can be programmed to tabulate and analyze the data.

Privacy and confidentiality must both be respected in record linkage studies. This is usually accomplished by requiring an oath of secrecy from all who handle the records involved. Each person in the world creates a book of life. This book starts with birth and ends with death. Its pages are made of the records of the principal events during the life course. Record linkage is the name given to the process of assembling the pages of this book into a volume.335

RECRUDESCENCE Reactivation of infection. See also relapse.

RECTANGULARIZATION OF MORTALITY The shape of survival curves as life expectancy increases: higher proportions of all who are born survive to old age and the graph becomes more “rectangular” in shape. Empirical observations in several countries have failed to demonstrate it, and the opposite was found in the United States, where the range of age at death was widening because of the impact of HIV disease and violence. See also compression of mortality.

RECURRENCE The second episode of a disease occurring after a first episode was considered cured. For instance, in tuberculosis, molecular techniques have shown that some recurrences are due to reinfection by a different strain rather than relapse with the same strain that had caused the first episode.336 Thus reinfection and relapse are two different causes of disease recurrence.

RECURRENCE RISK Risk of a second episode (and of subsequent episodes) of a disease. It provides information on the heterogeneity of risk in the population; it is thus useful for etiological studies. Observable in many areas of epidemiology, it is particularly accessible in the study of perinatal events. High recurrence rates of pregnancy problems may result from interactions between genetic causes and persistent environmental causes. Patterns of recurrence risk provide clues about the relative importance of genetic, epigenetic, and environmental factors; e.g., through comparisons of recurring pregnancy problems in women who change their male partner and women who keep the same partner.337

RECURRENT DISEASE A bacteriologically confirmed disease episode needing retreatment after a patient was successfully treated or defaulted during a previous disease episode.338 See also reinfection.

REDEFINING THE UNACCEPTABLE An expression to describe the history of public health. The public health advances when there is a combination of knowledge of the causes of public health problems, technical capability to deal with these causes, a sense of values that the health problems are important, and political will. It is the last of these that Vickers described as “redefining the unacceptable.”339

REDUCTION (of data)

  1. (Syn: “collapsing”) Reducing the number of categories of a set of data to simplify
    analysis.An important application is aggregation of small numbers and/or small areas in published tables from a national census in order to preserve the confidentiality of these localities and their residents.
  1. Formation of composite (derived) variables based on several originally collected variables, using methods ranging from simple indexes to factor analysis.
  1. Summarizing data by means of classification schemes and arithmetical manipulations.

REDUCTIONISM The philosophical concept of scientific investigation that is based on studying component parts of a system; e.g., proceeding from organs to tissues, to cells, to molecules. The reductionist view is that the whole can be explained in terms of the functioning of its parts. The discovery of the DNA structure can be viewed as a triumph for reductionist approaches. While some reductionist approaches have been fruitful, they may favor rigid compartmentalizing and fragmentation of sciences and hence delay progress.

They may also favor medicalization and genetization. Epidemiological studies that focus exclusively on individual risk factors are also a form of reductionism (not necessarily wrong or irrelevant), which tends to disregard contextual influences on health. Clinical and molecular epidemiology are practiced with both reductionist and integrative approaches.6,10,61–63 See also transdisciplinarity.

REED-FROST MODEL A mathematical model of infectious disease transmission and herd immunity developed by Lowell Reed (1886–1966) and Wade Hampton Frost (1880–1938). The model gives the number of new cases, C, of an infectious disease that can be expected in a closed, freely mixing population of immunes and susceptibles in time period t to t + 1, with varying assumptions about the distribution of each in the population:

where Ct+1 is the number of cases between time t and t + 1, St is the number of susceptibles at time t, and p is the probability that any specified individual will have contact with any other specified individual in the population. Elaborations of the model provide the theoretical basis for immunization programs that control infectious diseases without necessarily achieving 100% immunization coverage.340

REFERENCE POPULATION The standard against which a population that is being studied can be compared.

REFINEMENT The process of identifying new subcategories of study variables for the purpose of more accurate or more detailed description of relationships. An example is refinement of the concept of serum cholesterol level into high-, low-, and very low density lipoproteins.

REGISTER, REGISTRY In epidemiology the term register is applied to the file of data concerning all cases of a particular disease or other health-relevant condition in a defined population such that the cases can be related to a population base. With this information, incidence rates can be calculated. If the cases are regularly followed up, information on remission, exacerbation, prevalence, and survival can also be obtained. The register is the actual document and the registry is the system of ongoing registration.

In most developed countries all births and deaths are recorded through birth and death registration systems. Results and summaries are then tabulated and published. Examples of registries that have epidemiological value include the following:

Cancer registries, which secure reports of cancer patients as soon as possible after first diagnosis. The principal sources for these reports are the hospitals serving the community, but a few cases are not reported until death.
Twin registries, which have provided the basis for studies attempting to differentiate genetic from environmental factors in the etiology of cancer and other conditions where both genetic and environmental factors may be contributing causes.

Birth defect registries, which seek to document anomalies that are apparent at or soon after birth. They suffer from incompleteness owing to the omission of stillbirths and of anomalies that do not declare their presence until later in life, such as certain forms of congenital heart lesion, mental deficiency, and neurological disorders.

Many types of register—e.g., disease-specific, treatment-specific, “at risk,” local (hospital- or clinic-based)—are not population-based. Population-based registers are usually considered to be the most useful type for epidemiological purposes; clinic-based, disease-specific registers can be used as a source of cases for case-control studies.341

REGISTRATION The term registration implies something more than notification for the purpose of immediate action or to permit the counting of cases. A register requires that a permanent record be established, including identifying data. Cases may be followed up, and statistical tabulations may be prepared on both frequency and survival. In addition, the persons listed on a register may be subjects of special studies.

REGRESSAND In regression analysis, the variable whose mean values are studied in relation to regressors; the dependent variable.


  1. As used by Francis Galton (1822–1911), one of the founders of modern biology and biometry, in his book Hereditary Genius (1869), this meant the tendency of offspring of exceptional parents (unusually tall, unusually intelligent, etc.) to possess characteristics closer to the average for the general population. Hence “regression to the mean”; i.e., the tendency of individuals at the extremes to have values nearer to the mean on repeated measurement.
  1. In statistics, the relation of mean values of a dependent or regressand variable to independent or regressor variables (covariates).
  1. A synonym for regression analysis.

REGRESSION ANALYSIS Given data on a regressand (dependent variable) y and one or more regressors (covariates or independent variables) x1, x2, etc., regression analysis involves finding a mathematical model (within some restricted class of models) that adequately describes y as a function of the x’s, or that predicts y from the x’s. The most common form of model for an unbounded continuous y is a linear model; the logistic and proportional hazards models are the most common forms used when y is binary or a survival time, respectively.20

REGRESSION CURVE, LINE, SURFACE, PLANE Diagrammatic presentation of a regression model as a curve on a graph, usually drawn with the regressor, x, as the abscissa and the predicted average of the dependent variable (regressand), y, as ordi- nate. In the case of linear regression, the curve reduces to a line. A model with three variables (two regessors and one regressand) can be shown diagrammatically on a three-dimensional plot or stereogram; the result is a regression surface, which in the case of multiple linear regression reduces to a plane.

REGRESSION MODEL A mathematical model for the relation of the average value of a variable (the regressand) to other variables (the regressors). See also regression analysis.

REGRESSOR In regression analysis, a variable used to predict the regressand (depend- ent) variable. An independent variable or regression covariate.

REINFECTION A second infection by the same agent or a second infection of an organ with a different agent or strain. In tuberculosis, DNA fingerprinting of Mycobacterium tuberculosis showed that some recurrences are not treatment failures (i.e., they are not a relapse).342,343

REINFORCING FACTORS See causation of disease, factors in.


  1. Return of a disease state after remission or apparent cure.
  1. Insufficient bacteriological cure of a first episode. In tuberculosis the episode
    is caused by the same strain. In malaria, true relapses are caused by reactivation of dormant liver stage parasites (hypnozoites) found in Plasmodium vivax and P. ovale.131 See also recurrence.
  1. RELATIONSHIP See association.
  1. RELATIVE EFFECT A ratio of rates, proportions, or other measures of an effect. For example, the incidence rate ratio, calculated as the incidence rate in the exposed divided by the incidence rate in the unexposed, is a measure of relative effect.12
  1. RELATIVE EXCESS RISK (RER) A measure that can be used in comparisons of adverse reactions to drugs (or other exposures), based solely on the component of risk due to the exposure or drug under investigation, removing the risk due to background exposure experienced by all in the population:

where R1 is the rate in the study population. R2 is the rate in the comparison group, and R0 is the rate in the general population.344

RELATIVE ODDS See odds ratio.

RELATIVE POVERTY LEVEL The amount of income a person, family, or group needs to purchase a relative amount of basic necessities of life; these basic necessities are identified relative to each society and economy. See also absolute poverty level.


  1. The ratio of the risk of an event among the exposed to the risk among the unexposed; this usage is synonymous with risk ratio.
  1. The ratio of the incidence rate in the exposed to the incidence rate in the unexposed (i.e., the rate ratio). It is not synonymous with odds ratio (OR). In some biostatistical articles, it has been used for the ratio of forces of morbidity. The use of relative risk for OR arises from the fact that for “rare” (infrequent) diseases, the two quantities may approximate one another. For common occurrences (e.g., neonatal mortality in infants under 1500-g birth weight), the approximations do not hold.
  1. Let ARC be the absolute risk of events in the control group and ART the absolute risk of events in the treatment group; then RR = ARC / ART = 1 – RR reduction (RRR). Example: an RR of 0.7 equals an RRR of 0.3, i.e., a 30% reduction in the RR of the outcome in the treatment group compared with the control group. See also absolute risk (AR); absolute risk reduction (ARR); control group; cumulative incidence ratio; risk difference.


  1. The difference in event rates between two groups expressed as a proportion of the event rate in the untreated group. The RRR may be similar in populations with different risks. An estimate of the number of people spared the consequences of an exposure that has been eliminated or controlled. The amount by which a person’s risk of disease is reduced by elimination or control of an exposure to risk.
  1. Let ARC be the absolute risk of events in the control group and ART the absolute risk of events in the treatment group; then RRR = (ARC – ART) / ARC = 1 – Risk Ratio.14,15 See also absolute risk (AR); absolute risk reduction (ARR); number needed to treat (NNT).
  1. The RRR, ARC, and NNT are related as follows: NNT × RRR × ARC = 1. This equation is used to assess plausible benefits of an intervention in populations and individuals with different levels of baseline risk.298


  1. The importance for existing ideas or practices. The degree to which a study, program, policy, or organization should theoretically change or can actually influence knowledge, beliefs, ideas, attitudes, decisions, actions, policies, structures, proce- dures, techniques, or processes of all sorts (social, cultural, political, organizational, individual, medical, biological, etc.).
  1. In epidemiology, a relevant study or program may be one that makes a practical or a theoretical contribution to the identification, characterization, understanding, or solution of a public health, environmental, social, clinical, biological, or technological problem. Epidemiological research usually aims at having social, environmental, or public health relevance; epidemiological studies often also have clinical, biological, methodological, or technological relevance.
  1. In clinical and epidemiological research, relevance is commonly used as a synonym of importance and of significance. Statistical significance is always distinguished from clinical and public health significance. A statistically significant effect (e.g., with a P < 0.01) may be found in a study with a large number of participants and yet lack clinical or public health significance (because the magnitude of the effect is small, for instance). Hence, statistical significance never equals significance, and significance encompasses more than statistical significance. Clinical studies usually aim at being clinically significant, important, or relevant for the care of patients. They are often mechanistically relevant; e.g., they produce knowledge on mechanisms of disease or of drug action. The health sciences scientific literature contains several thousand articles with the word relevance in the title and over 200,000 with relevance or relevant in the abstract. See also significance, clinical; significance, public health; significance, statistical.

RELIABILITY The degree of stability exhibited when a measurement is repeated under identical conditions. Reliability refers to the degree to which the results obtained by a measurement procedure can be replicated. Lack of reliability may arise from diver- gences between observers or instruments of measurement or instability of the attribute being measured. See also measurement, terminology of; observer variation.

REMOTE SENSING The collection and interpretation of information at a distance from the phenomenon or object being observed, e.g., by aerial photography, satellite imag- ing. Remote sensing has provided valuable information about ecological zones hospi- table to mosquitoes and other vectors, plankton blooms that can potentiate cholera outbreaks, etc.

REPEATABILITY (Syn: reproducibility) A test or measurement is repeatable if the results are identical or closely similar each time it is conducted. See also measurement, termi- nology of; reliability.

REPLACEMENT-LEVEL FERTILITY The level of fertility at which a cohort of women are having only enough daughters to replace themselves in the population. By defini- tion, it is equal to a net reproduction rate of 1.00. The total fertility rate is also used as a measure of replacement level fertility. In the United States today, a total fertility rate of 2.12 is considered to be replacement level; it is higher than 2 because of mortality and because of a sex ratio greater than 1 at birth. The higher the female mortality rate, the higher is the replacement-level fertility. See also gross reproduction rate.

REPLICATION The execution of an experiment or survey more than once so as to confirm the findings, increase precision, and obtain a closer estimation of sampling error.

Exact replication should be distinguished from consistency of results on replication. Exact replication is often possible in the physical sciences, but in the biological and behavioral sciences, to which epidemiology belongs, consistency of results on replica- tion is often the best that can be attained. Consistency of results on replication is per- haps the most important criterion in judgments of causality.

REPORTING BIAS Selective revealing or suppression of information (e.g., about past medical history, smoking, sexual experiences).

REPRESENTATIVE SAMPLE The term representative as it is commonly used is unde- fined in the statistical or mathematical sense; it means simply that the sample resembles the population in some way.98 The use of probability sampling will not ensure that any single sample will be “representative” of the population in all possible respects. If, for example, it is found that the sample age distribution is quite different from that of the population, it is possible to make corrections for the known differences. A common fallacy lies in the unwarranted assumption that, if the sample resembles the population closely on those factors that have been checked, it is “totally representative” and that no difference exists between the sample and the universe or reference population.

Some confusion arises according to whether representative is regarded as meaning “selected by some process which gives all samples an equal chance of appearing to represent the population” or, alternatively, whether it means “typical in respect of certain characteristics, however chosen.” On the whole, it seems best to confine the word representative to samples that turn out to be so, however chosen, rather than applying it to those chosen with the objective of being representative.98 See also general population; validity, study.

REPRESSION BIAS Failing to pursue a line of enquiry because the enquiry fails to con- form to prevailing dominant social or research paradigms. It may lead to publication bias. It undermines public health because it delays the discovery of scientific knowledge on health risks and compromises credibility in science and administrative processes for assessing and preventing exposure to risks. See also suppression bias; scientific misconduct.

REPRODUCIBILITY See repeatability.

REPRODUCTIVE ISOLATION Absence of interbreeding between populations.

REPRODUCTIVE SUCCESS In population genetics, quantitatively, the proportion of offspring surviving long enough to reproduce.

REPROGRAMMING In genetics and epigenetics, the erasure and reestablishment of DNA methylation during mammalian development. After fertilization, the paternal and maternal genomes are once again demethylated and remethylated. This repro- gramming might be required for totipotency of the newly formed embryo and erasure of acquired epigenetic changes. See also epigenetic inheritance.

RESCUE BIAS A form of interpretive bias that occurs in discounting data by finding selective faults in a study when the data are viewed unfavorably or by discounting faults when the data are viewed favorably. A deliberate attempt to evade evidence that con- tradicts expectation or interests.34,259 See also auxiliary hypothesis bias.

RESEARCH A class of activities designed to develop or contribute to generalizable knowledge; generalizable knowledge consists of theories, principles, or relationships or the accumulation of information on which these are based that can be corroborated by acceptable scientific methods of observation, inference, and/or experiment (adapted from the Council for International Organizations of the Medical Sciences, 1993).

When humans are the subjects of epidemiological research, ethical review is man- datory; however, there is a blurry boundary between research, which must undergo review, and common clinical or public health practice (e.g., surveillance and epidemic control), to which the same rules may not apply. See also integrative research.

RESEARCH DESIGN The “architecture” of a study: its structure, specific details of the studied population, time frame, method, and procedures, including ethical considera- tions, all of which should be explicitly stated in a research protocol. Details of all aspects of research design are essential to anyone seeking to replicate a study, so there is a moral obligation to ensure that these details are in the public domain. They must be adhered to by all centers in a multicenter study.

RESEARCH ETHICS BOARD, COMMITTEE See institutional review board.

RESEARCH SUBJECT A person who is studied. Under some circumstances the word subject is perceived as demeaning, and other terms may be more socially acceptable, e.g., study participant, volunteer, or patient.


  1. Any person, animal, arthropod, plant, soil, or substance, or combination of these in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host.
  1. The natural habitat of the infectious agent.

RESILIENCE A process of positive adaptation in the face of adversity; e.g., intrinsic and extrinsic factors confer educational, emotional, and behavioral resilience to children.16

RESIDUAL CONFOUNDING Confounding that persists after unsuccessful attempts to adjust for it. Also referred to as unmeasured confounding, although the problem lies with unmeasured or poorly measured confounders. Main sources of residual confounding are insufficiently detailed information, improper categorization, and misclassification of one or more confounding variables. It is an outcome-specific concept.82,253 See also adjust- ment; confounding bias; inverse probability weighting; standardization.


  1. The capacity of a system to distinguish between truly distinct things that are close together.
  1. A component of a measuring instrument that helps determine precision. The degree of refinement of the measuring process is commonly referred to as the “resolution” or the “resolving power of the system.” See also power. The capability of distinguishing between things that are indeed separate or distinct from one another.

RESOURCE ALLOCATION The process of deciding how to distribute financial, mate- rial, and human resources among competing claimants for these resources. Resource allocation is an essential feature of all health planning everywhere.345 Epidemiologi- cal evidence on need, demand, supply, and use of existing services is integral to the process, although factors such as political, commercial, and emotional considerations sometimes carry more weight than objective epidemiological evidence; ethical consid- erations should affect decisions about resource allocation.

RESPONSE BIAS Systematic error due to differences in characteristics between those who choose or volunteer to take part in a study and those who do not.

RESPONSE RATE The number of completed or returned survey instruments (question- naires, interviews, etc.) divided by the total number of persons who would have been surveyed if all had participated. Usually expressed as a percentage. Nonresponse can have several causes, e.g., death, removal from the survey community, and refusal. See also bias; completion rate; nonparticipants.

RETROLECTIVE Pertaining to data gathered without planning for the needs of an investigation. See also prolective, also a term suggested by A. R. Feinstein.321,322 See also directionality.

RETROSPECTIVE STUDY A research design used to test etiological hypotheses in which inferences about exposure to the putative causal factor(s) are derived from data relating to characteristics of the persons under study or to events or experiences in their past. The essential feature is that some of the persons under study have the dis- ease or other outcome condition of interest, and their characteristics and past experiences are compared with those of other, unaffected persons. Persons who differ in the severity of the disease may also be compared. It is no longer considered a synonym for case-control study.

RETROVIRUS This name is given to a family of RNA viruses characterized by the pres- ence of an enzyme, reverse transcriptase, that enables transcription of RNA to DNA inside an affected cell. Thus retroviruses can make copies of themselves in host cells. The most important retrovirus is the human immunodeficiency virus (HIV); this makes copies of itself in host cells, such as T4 “helper” lymphocytes, and normal immune responses are disrupted.

REVERSE TRANSCRIPTION The process by which an RNA molecule is used as a template to make a single-stranded DNA copy.243 This is the mode of action of the human immunodeficiency virus when it attacks T4 helper lymphocytes, which maintain immune competence.

REVES Réseau Espérances de Vie en Santé ([International] Network on Health Expect- ancy and the Disability Process) (

REVIEW BIAS In diagnostic accuracy studies, bias that occurs when the investigator knows the results of the new diagnostic test when the “gold standard” test is interpreted or when the investigator knows the results of the gold standard test when the new diag- nostic test in interpreted.

REVIEW, SYSTEMATIC The application of strategies that limit bias in the assembly, critical appraisal, and synthesis of all relevant studies on a specific topic. meta-analysis may be, but is not necessarily, used as part of this process. Systematic reviews focus on peer-reviewed publications about a specific health problem and use rigorous, standard- ized methods for selecting and assessing articles. A systematic review differs from a meta-analysis in not including a quantitative summary of the results.106,280,286,327,330

RIDIT A method of presenting observed values, e.g., health measurement scale scores of a group, relative to a reference population.345 The average ridit for the group shows the probability that a member of the group differs from a member of the reference population. For example, if the average ridit for a group is 0.62, 62% of persons in the reference population have higher scores than a randomly chosen member of the group.

RIDIT ANALYSIS A method proposed by Bross (1958) for analyzing subjectively catego- rized or poorly recorded data. It consists of allocating scores relative to the identified distribution of the data based upon a transformation to the uniform distribution rather than the normal distribution.

RISK The probability that an event will occur, e.g., that an individual will become ill or die within a stated period of time or by a certain age. Also, a nontechnical term encompassing a variety of measures of the probability of a (generally) unfavorable outcome. See also probability.


  1. The qualitative or quantitative estimation of the likelihood of adverse effects that may result from exposure to specified health hazards or from the absence of beneficial influences. Risk assessment uses clinical, epidemiologic, toxicologic, environmental, and any other pertinent data.
  1. The process of determining risks to health attributable to environmental or other hazards. The process consists of four steps:
    Hazard identification: Identifying the agent responsible for the health problem, its adverse effects, the target population, and the conditions of exposure.
    Risk characterization: Describing the potential health effects of the hazard, quantifying dose-effect and dose-response relationships.
    Exposure assessment: Quantifying exposure (dose) in a specified population based on measurement of emissions, environmental levels of toxic substances, biological monitoring, etc.
    Risk estimation: Combining risk characterization, dose-response relationships, and exposure estimates to quantify the risk level in a specific population. The end result is a qualitative and quantitative statement about the health effects expected and the proportion and number of affected people in a target population, including estimates of the uncertainties involved. The size of the exposed population must be known.

RISK-BENEFIT ANALYSIS The process of analyzing and comparing on a single scale the expected positive (benefits) and negative (risks, costs) results of an action or lack of an action.

RISK-BENEFIT RATIO The results of a risk-benefit analysis expressed as the ratio of risks to benefits.

RISK CHARACTERIZATION See risk assessment.

RISK DIFFERENCE (RD) (Syn: absolute risk reduction) The absolute difference between two risks: one minus the other. See also relative risk reduction.

RISK ESTIMATION See risk assessment.

RISK EVALUATION See risk management.

RISK FACTOR (Syn: risk indicator)

  1. An aspect of personal behavior or lifestyle, an environmental exposure, or an inborn or inherited characteristic that, on the basis of scientific evidence, is known to be associated with meaningful health-related condition(s). In the twentieth century multiple cause era, a synonymous with determinant acting at the individual level.
  1. An attribute or exposure that is associated with an increased probability of a specified outcome, such as the occurrence of a disease. Not necessarily a causal factor: it may be a risk marker.
  1. A determinant that can be modified by intervention, thereby reducing the probability of occurrence of disease or other outcomes. It may be referred to as a modifiable risk factor, and logically must be a cause of the disease.

The term risk factor became popular after its frequent use by T. R. Dawber and others in papers from the Framingham study.346 The pursuit of risk factors has motivated the search for causes of chronic disease over the past half-century. Ambiguities in risk and in risk-related concepts, uncertainties inherent to the concept, and different legitimate meanings across cultures (even if within the same society) must be kept in mind in order to prevent medicalization of life and iatrogenesis.124–128,136,142,240

RISK MANAGEMENT The steps taken to alter (i.e., reduce) the levels of risk to which an individual or a population is subject. The managerial, decision-making, and active hazard control process to deal with environmental agents of disease, such as toxic substances, for which risk evaluation has indicated an unacceptably high level of risk.

The process consists of three steps:

  1. risk evaluation: Comparison of calculated risks or public health impact of exposure
    to an environmental agent with the risks caused by other agents or societal factors and with the benefits associated with the agent as a basis for deciding what is an acceptable risk.
  1. exposure control: Actions taken to keep exposure below an acceptable maximum limit.
  1. risk monitoring: The process of measuring reduction in risk after exposure control actions have been taken in order to reassess risks and initiate further control measures if necessary.

RISK MARKER (Syn: risk indicator) An attribute that is associated with an increased probability of occurrence of a disease or other specified outcome and that can be used as an indicator of this increased risk. Not necessarily a causal factor. See also risk factor.

RISK MONITORING See risk management

RISK RATIO The ratio of two risks, usually exposed/not exposed.


  1. A property of a statistical test or procedure that confers to it a certain degree of insensitiveness to departures from the assumptions from which it is derived (e.g., that the data are normally distributed).
  1. The resistance of genes to manipulations supposed to lead to a predicted phenotype. Essentially due to the fundamental regulatory role of interactions among genes, and to a common redundancy of functions and regulatory mechanisms converging towards a specific goal.207,347

ROUNDING The process of eliminating surplus digits, taking the nearest whole number, multiple of 10, etc., as an approximation of the value of a measurement. See also digit preference.

RUBRIC Section or chapter heading. Used in epidemiology with reference to groups of diseases, e.g., in the International Classification of Disease (ICD).