D

DAG See directed acyclic graph.

DALY See disability-adjusted life years.

DATA A collection of items of information. Note: The singular of data is datum; the plural noun should not be accompanied by a singular verb.

DATA CONTAMINATION See contamination, data.

DATABASE An organized set of data or collection of files that can be used for a specified purpose.

DATA CLEANING The process of excluding the information in incomplete, inconsistent records or irrelevant information collected in a survey or other form of epidemiological study before analysis begins. This may mean excluding information that would distort the results if an attempt were made to edit and include it in the analysis, but it can also introduce biases. The fact that this step has been taken should be reported, along with the results of the study of analyzed data. See also raw data.

DATA DREDGING A jargon term meaning analyses done on a post hoc basis without benefit of prestated hypotheses as a means of identifying noteworthy differences. Such analyses are sometimes done when data have been collected on a large number of vari- ables and unanticipated hypotheses are suggested by hypothesis-free analyses. The sci- entific validity of data dredging is dubious, usually unacceptable. See also chance; cross validation; overfitting; random.

DATA MINING The extraction of information from large databases or files, often with the use of artificial intelligence technology and sophisticated statistical and bioinformat- ics methods. An important exploratory approach in “–omics” research (e.g., genomics, peptidomics).

DATA PROCESSING Conversion of items of information into a form that permits storage, retrieval, and analysis.

DATA REDUCTION The process of summarizing a set or sets of data in the form of an index, such as life expectancy or gross domestic product.

DEATH CERTIFICATE A vital record signed by a licensed physician or another designated health worker that includes cause of death, decedent’s name, sex, birth date, places of residence and of death, and whether the deceased had been medically attended before death. Occupation, birthplace, and other information may be included. Immediate cause of death is recorded on the first line, followed by conditions giving rise to the immediate cause; the underlying cause is entered last.

The underlying cause is coded and tabulated in official publications of cause-specific mortality. Other significant conditions may also be recorded separately, as is the mode of death, whether accidental or violent, etc. The most important entries on a death certi- ficate are underlying causes of death and cause of death. These are defined in the tenth (1990) revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10) as follows:

Causes of death: The causes of death to be entered on the medical certificate of cause of death are all those diseases, morbid conditions, or injuries that either resulted in or contributed to death and the circumstances of the accident or violence which produced any such injuries.

Underlying cause of death: The underlying cause of death is (1) the disease or injury that initiated the train of events leading to death or (2) the circumstances of the accident or violence that produced the fatal injury.”

Personal identifying information such as birthplace, parents’ names (last name at birth), birth date, and personal identifying numbers are included on death certificates in some jurisdictions; this extra information makes possible a range of record linkage studies. See also International Form of Medical Certificate of Causes of Death.

DEATH REGISTRATION AREA A geographic area for which mortality data are collected, and often published.

DECISION ANALYSIS A derivative of operations research and game theory to identify all available choices, and potential outcomes of each, in a series of decisions that have to be made (e.g., about aspects of patient care as diagnostic procedures, preventive and therapeutic regimens, prognosis). Epidemiological data play a large part in analyzing the probabilities of outcomes following each potential choice. The range of choices can be plotted on a decision tree; at each branch of the tree, or decision node, the prob- abilities of each outcome that can be predicted are displayed. The decision tree thus portrays the choices available to those responsible for patient care and the probabilities of each outcome that will follow the choice of a particular action or strategy. The relative worth of each outcome is preferably described as a utility or quality-of-life mea- sure (e.g., a probability of life expectancy or of freedom from disability, often expressed as quality-adjusted life years, or QALYs). See also clinical decision analysis.

DECISION TREE The alternative choices expressed, in quantitative terms, available at each stage in the process of thinking through a problem may be likened to branches and the hierarchical sequence of options to a tree. Hence, decision tree. It is a graphic device used in decision analysis in which a series of decision options are represented as branches and subsequent possible outcomes are represented as further branches. The decisions and the eventualities are presented in the order in which they are likely to occur. The junction where a decision must be taken is called a decision node.

DECOMPOSITION METHOD Comparison of groups by analyzing mathematical functions of rates, incidence densities, and exposure prevalence. This simplifies identification of relevant contributing factors in risk analysis.110

DEDUCTION Reasoned argument proceeding from the general to the particular.

DEDUCTIVE LOGIC Logic that predicts specific outcomes from prior general hypotheses— that is, it proceeds from the general to the particular. Logic based on derivation of necessary conclusions implied by explicit assumptions (premises), as in mathematics. See also hypothetico-deductive method; inductive logic.


DEGREES OF FREEDOM (df) The number of independent comparisons that can be made between the members of a sample. Roughly, it refers to the number of inde- pendent dimensions of variation in a sample under an assumed sampling model. More specifically, it is the number of independent contributions to a sampling distribution (such as χ2, t, and F distribution) from the data-generating mechanism. For example, in a contingency table with fixed margins, the total degrees of freedom is one less than the number of row categories multiplied by one less than the number of column categories. See also dimensionality.


DELPHI METHOD Iterative circulation to a panel of experts of questions and responses that are progressively refined in light of responses to each round of questions; prefer- ably participants’ identities should not be revealed to each other. The aim is to reduce the number of viable options or solutions, perhaps to arrive at a consensus judgment on an issue or problem, or a set of issues or problems, without allowing any one participant to dominate the process. The method was developed at the RAND Corporation.

DEMAND (FOR HEALTH SERVICES) Willingness and/or ability to seek, use, and in some settings, pay for services. Sometimes further subdivided into expressed demand (equated with use) and potential demand or need.

DEMOGRAPHIC TRANSITION The evolution from high to low fertility and mortality rates in a country. Formerly thought to be related to technological change and industrialization but probably more directly caused by improvements in female literacy and the status of women. It is accompanied by a change in the age composition of the population as birthrates and death rates decline; usually infant and child mortality rates decline as well. As a result there is a decrease in the proportion of children and young adults and an increase in the proportion of older persons in the population, i.e., an aging of the population. See also ecological transition; “epidemiological transition” theory.

DEMOGRAPHIC ENTRAPMENT A community may be said to be demographically trapped if (1) it exceeds the carrying capacity of its local ecosystem; (2) there is no other land to which it can migrate; and (3) it has too few exports to exchange for food and other essentials. Common outcomes include poverty, stunting, starvation, and violence. There may be a warning stage, during which starvation or violence can be expected (e.g., because population is increasing rapidly).

DEMOGRAPHY The study of populations, especially with reference to size and density, fertility, mortality, growth, age distribution, migration, and vital statistics, and the interaction of all these with social and economic conditions.

DEMONSTRATION MODEL An experimental health care facility, program, or system with built-in provision for measuring aspects such as costs per unit of service, rates of use by patients or clients, and outcomes of encounters between providers and users. The aim usually is to determine the feasibility, efficacy, effectiveness, and/or efficiency of the model service.

DENOMINATOR The lower portion of a fraction, used to calculate a rate or ratio. The population (or population experience, as in person-years, passenger-miles, etc.) at risk in the calculation of a rate or ratio. Valid information on denominators is essential in clinical and epidemiological research and also in many public health activities. See also numerator.

DENSITY CASE-CONTROL STUDY A variant of the case-control design in which the controls are drawn in such a way that they represent the person-time experience that generated the cases, usually by density sampling. This design provides an estimate of the rate ratio with no rare disease assumption. A type of case-base study. See also case-cohort study.

DENSITY-EQUALIZING MAP See isodemographic map.

DENSITY OF POPULATION Demographic term meaning numbers of persons in relation to available space.

DENSITY SAMPLING A method of selecting controls in a case-control study in which cases are sampled only from incident cases over a specific time period and controls are sampled and interviewed throughout that period (rather than simply at one point in time, such as the end of the period). This method can reduce bias due to changing expo- sure patterns in the source population and allows estimation of the rate ratio without any rare-disease assumption.

DEONTOLOGICAL A duty-based theoretical approach to ethics. Right actions stem from freely embraced obligations to universal moral imperatives, such as the obligation to respect persons as ends and not as means.36

DEPENDENCY RATIO Ratio of children and old people in a population in comparison to all others (i.e., the proportion of economically inactive to economically active); “children” are usually defined those below 15 years of age and “old people” as those 65 years of age and above.

DEPENDENT VARIABLE

1. A variable the value of which is dependent on the effect of other variable(s)— independent variable(s)—in the relationship under study. A manifestation or outcome whose variation we seek to explain or account for by the influence of independent variables.

2. In regression analysis, the variable whose average value is being studied in relation to regressors (covariates or “independent” variables).20 See also independent variable; regressand; regressor.

DESCRIPTIVE EPIDEMIOLOGY Epidemiological studies and activities (e.g., surveil- lance) whose descriptive components are much stronger than their analytic components or that clearly fall within the descriptive area of the descriptive-analytic spectrum. Descriptive study of the occurrence of disease and other health-related characteristics in human populations. General descriptions concerning the relationship of disease to basic characteristics such as age, gender, race, occupation, social class, and geographic location; even such general descriptions may have analytic dimensions. The major characteristics in descriptive epidemiology can be classified under the headings persons, place, and time. Descriptive epidemiology is always observational, never experimental; hence observational epidemiological studies may be descriptive; nevertheless, epide- miological research studies are often analytic. See also analytic study; case reports; etiological study; observational study.

DESCRIPTIVE STUDY A study concerned with and designed only to describe the existing distribution of variables without much regard to causal relationships or other hypotheses. An example is a community health survey used to determine the health status of the people in a community. In a descriptive study, a parameter of disease occurrence is related to a determinant without concern for a causal interpretation of the relation.5 Descriptive studies (e.g., analyses of population registries) can be used to measure risks or trends in health indicators, generate hypotheses, monitor public health policies, etc. Contrast analytic study; etiological study.

DESIGN See research design.

DESIGN BIAS The difference between a true value and that obtained as a result of faulty design of a study. Examples include uncontrolled studies where the effects of two or more processes cannot be separated for lack of measurement of key causes of the expo- sure or outcome (confounding); also studies done on poorly defined populations or with unsuitable control groups.

DESIGN EFFECT A bias in study findings attributable to the study design. A specific form is bias attributable to intraclass correlation in cluster sampling. The design effect for a cluster design is the ratio of the variance for that design to the variance calculated from a simple random sample of the same size.

DESIGN VARIABLE

  1. A study variable whose distribution in the subjects is determined by the investigator.
  2. In statistics, a variable taking on the value 1 to indicate membership in a particular category and 0 or –1 to indicate nonmembership in the category. Used primarily in analysis of variance. See also indicator variable.

DESMOTERIC MEDICINE The practice of medicine in a prison. Derived from the Greek desmoterion, prison.

DETECTABLE PRECLINICAL PERIOD The period between the time when a disease is capable of yielding a positive screening test and the appearance of clinical symptoms and/or signs. See also lead time.

DETECTION BIAS Bias due to systematic differences between the study groups in ascertainment, assessment, diagnosis, or verification of outcomes. As with other biases, there are many mechanisms and forms of detection bias.10,12,14,31 An example is verification of diagnosis by laboratory tests in hospital cases but failure to apply the same tests to cases outside the hospital.

DETERMINANT Any factor that brings about change in a health condition or other defined characteristic. Single specified causes. A determinant makes a difference to a given outcome. Does not imply a deterministic philosophy of health. See also causality; diseases of complex etiology; mathematical model.

DETERMINANT, DISTAL (DISTANT) (Syn: upstream determinant) A causal factor that is remote or far apart in position or time to the outcome of concern, making it more difficult to discern or trace within the causal pathway than other causal factors that are less far away from the outcome.71,72 An example is atmospheric contamination with ozone-destroying substances that increase the risk of skin cancer.111 In infectious dis- eases, the microbial agent is more proximal to the disease than social factors such as poverty, which are more distant or upstream but no less influential on the individual risk of developing the disease. See also strategy, “population.”

DETERMINANT, PROXIMAL (PROXIMATE) (Syn: downstream determinant) An estab- lished or postulated risk factor that is nearer in time or distance before an outcome of concern. The causal pathway is clearly enough defined to allow confident assertion of linkage between the determinant and the outcome. See also strategy, “high-risk.”

DETERMINISM, GENETIC A view of genetics according to which genetic inheritance not only influences but strongly constrains human development, health, and behav- ior. It disregards environmental influences on gene expression and social influences on health states. See also epigenetic inheritance; genetic penetrance; monogenic diseases; polygenic diseases.

DETERMINISTIC METHOD A method that predicts outcomes perfectly, without allowance for statistical (chance) variation.

DETERMINISTIC MODEL A representation of a system, process, or relationship in mathematical form in which relationships are fixed (i.e., they take no account of probability and chance), so that any given input invariably yields the same result.6,10–12 See also mathematical model.

DETERMINANT OF FERTILITY, PROXIMATE Factor having a direct influence on fertility, such as contraceptive use, age at marriage, age at first sexual intercourse, breast- feeding, or abortion.

DEVELOPMENTAL AND LIFE-COURSE EPIDEMIOLOGY The study of long-term effects on later health or disease risk of physical or social exposures during gestation, childhood, adolescence, young adulthood, and later adult life. The premise is that various biological and social factors throughout life independently, cumulatively, and inter- actively influence health and disease in adult life. The aim is to elucidate biological, behavioral, and psychosocial processes that operate across an individual’s life course or across generations to influence disease risk.16,112,113

DEVELOPMENTAL ORIGINS HYPOTHESIS The hypothesis that cardiovascular disease and type 2 diabetes originate through developmental plasticity in response to under- nutrition. A hypothesis proposed in 1990 by the British epidemiologist David Barker (b. 1939) that intrauterine growth retardation, low birth weight, and premature birth have a causal relationship to the origins of hypertension, coronary heart disease, and non–insulin dependent diabetes in middle age.114,115 As growth during infancy and early childhood is also linked to later disease, developmental origins hypothesis is preferred to fetal origins hypothesis.45 The hypothesis is evolving to include evidence that exposure to environmental factors early in development involves epigenetic modifications, such as DNA methylation, which influence adult disease susceptibility;116,117 e.g., in utero or neonatal exposure to bisphenol A (BPA), a high-production-volume chemical used in the manufacture of polycarbonate plastic, may be associated with higher body weight, increased risk of breast and prostate cancer, and altered reproductive function.118 See also thrifty phenotype.

DIAGNOSIS The process of determining health status and the factors responsible for producing it; may be applied to an individual, family, group, or community. The term is applied both to the process of determination and to its findings. See also disease label; semiology.

DIAGNOSIS-RELATED GROUP (DRG) Classification of hospital patients according to diagnosis and intensity of care required, used by insurance carriers to set reimbursement scales.

DIAGNOSTIC AND STATISTICAL MANUAL OF MENTAL DISORDERS (DSM) A manual that aims to systematize and standardize the definitions of mental disorders developed by the American Psychiatric Association. It contains a listing of psychiatric disorders and their corresponding diagnostic codes; each disorder is accompanied by a set of diagnostic criteria and text containing information about the disorder (associated features; prevalence; familial patterns; age-, culture-, and gender-specific features; and differential diagnosis). No information about treatment or presumed etiology is included. It is used by mental health professionals from a variety of disciplines for clinical, research, administrative, and educational purposes. DSM-IV is the fourth edition, published in 1994. The DSM-V is currently in preparation.

DIAGNOSTIC INDEX A system for recording diagnoses, diseases, or problems of patients or clients in a medical practice or service, usually including identifying information (name, birthdate, sex) and dates of encounters.

DIAGNOSTIC SUSPICION BIAS A bias that may occur when knowledge of the subject’s prior exposure to a putative cause (ethnicity, drug intake, a second disorder, an environmental exposure) influences both the intensity and the outcome of the diagnostic process.14,63

DICHOTOMOUS SCALE See measurement scale.
DIFFERENTIAL The difference(s) shown in tabulation of health and vital statistics according to age, sex, or some other factor; age differentials are the differences revealed in the tabulations of rates in age groups, sex differentials are the differences in rates between males and females, income differentials are differences between designated income categories, etc.

DIFFUSION THEORY

  1. The concept that infectious pathogens and ideas diffuse through a population.119
  2. Theories explaining the dissemination of ideas and customs to other populations.

3. The “innovation-diffusion theory” explains how innovative ideas spread through segments of society, including the role of opinion leaders and the media.

DIGIT PREFERENCE A preference for certain numbers that leads to rounding off meas- urements. Rounding off may be to the nearest whole number, even number, multiple of 5 or 10, or (when time units like a week are involved) 7, 14, etc. This can be a form of observer variation or an attribute of respondent(s) in a survey.

DIMENSIONALITY The number of dimensions (i.e., scalar quantities) needed for accurate description of an element of a vector space. See also degrees of freedom.

DIRECT ADJUSTMENT, DIRECT STANDARDIZATION See standardization.

DIRECTED ACYCLIC GRAPH (DAG) See causal diagram.

DIRECTION Blalock’s120 synonym for David Hume’s connection.71 It indicates a linkage from cause to effect that is repeatedly demonstrable, hence predictable.

DIRECTIONALITY

1. The direction of inference of a study.12,31,121 It may be retrospective (backward- looking) or prospective (forward-looking).

2. The sign of a relationship between variables. Correlation coefficients are directional measures of association because the sign changes if one of the variables is reversed.

DIRECTIVES See guidelines.

DIRECT OBSTETRICAL DEATH See maternal mortality.

DISABILITY Temporary or long-term reduction of a person’s capacity to function. See also International Classification of Impairments, Disabilities, and Handicaps for the official WHO definition.

DISABILITY-ADJUSTED LIFE YEARS (DALYs) A DALY lost is a measure of the burden of disease on a defined population. It is hence an indicator of population health. DALYs are advocated as an alternative to quality-adjusted life years (QALYs). They are based on adjustment of life expectancy to allow for long-term disability as esti- mated from official statistics; the necessary data to do so may not be available in some areas. The concept postulates a continuum from disease to disability to death that is not universally accepted, particularly by the community of persons with disabilities. DALYs are calculated using a “disability weight” (a proportion less than 1) multiplied by chronological age to reflect the burden of the disability. DALYs can thus produce estimates that accord greater value to fit than to disabled persons and to the middle years of life rather than to youth or old age.122 See also disability-free life expectancy.

DISABILITY-FREE LIFE EXPECTANCY (Syn: active life expectancy) The average number of years an individual is expected to live free of disability if current patterns of mortality and disability continue to apply.123 A statistical abstraction based on existing age-specific death rates and either age-specific disability prevalences or age-specific dis- ability transition rates.

DISASTER EPIDEMIOLOGY The application of epidemiological principles and tools to managing emergency public health programs (e.g., to reduce morbidity and mortality among displaced populations).

DISCLOSURE OF INTERESTS

  1. In health sciences research and other professional activities (e.g., lecturing, consulting), the action of making researchers’ interests on a given issue known. Following requirements of the International Committee of Medical Journal Editors (www.icmje.org), all participants in the peer review and publication process must disclose all relationships that could be viewed as presenting a potential conflict of interest. Editors may use information disclosed in conflict-of-interest and financial interest statements as a basis for editorial decisions. Editors should publish this information if they believe it is important in judging the manuscript. Editors should also publish regular disclosure statements about potential conflicts of interest related to the commitments of journal staff. A similar rationale is applied by governments that receive expert advice from health scientists. See also conflict of interest.
  2. In certain types of law, the obligation that each party has to the other parties to reveal or make known all facts relevant to the subject matter of the contract.

The provision of financial and other types of information concerning a company to those with an interest in the economic activities of the company.

DISCORDANT A term used in twin studies to describe a twin pair in which one twin exhibits a certain trait and the other does not. Also used in matched-pair case- control studies to describe a pair whose members had different exposures to the risk factor under study. Under conventional analytical methods, only the discordant pairs are informative about the association between exposure and disease.


DISCOUNT RATE A measure of costs, benefits, and outcomes in relation to time that allows for the fact that money (and health) have greater value in the present than at some future time. A term used mainly in economics and in clinical decision analysis.

DISCRETE DATA Data that can be arranged into naturally occurring or arbitrarily selected groups or sets of values as opposed to data in which there are no naturally occur- ring breaks in continuity (i.e., continuous data). An example is number of decayed, missing, and filled teeth (DMF).

DISCRIMINANT FUNCTION ANALYSIS A statistical analytical technique used with discrete dependent variables; it is concerned with separating sets of observed values and allocating new values and can sometimes be used instead of logistic regression analysis. Kendall and Buckland98 refer to this as “discriminatory analysis” and describe it as a rule for allocating individuals or values from two or more discrete populations to the correct population with minimal probability of misclassification.

DISEASE

  1. Literally, dis-ease, the opposite of ease, when something is wrong with a bodily function.
  2. The words disease, illness, and sickness are sometimes used as if they were loosely interchangeable, but they are better regarded as not synonymous: 22,124–126
    i. Disease is the biological dimension of nonhealth, an essentially physiological dysfunction.
    ii. Illness is a subjective or psychological state of the person who feels aware of not being well; the experience of a person with a disease; a social construct fashioned out of transactions between healers and patients in the context of their common culture.
    iii. Sickness is a state of social dysfunction of a person with a disease; the role that the individual assumes when ill; a result of being defined by others as “unhealthy.”

In the real world, lay concepts of illness and medical concepts of disease interact and shape each other. Neither disease nor illness is infinitely malleable: both are constrained by bio- logy and by culture.127,128 See also disorder; embodiment; semiology; sickness “career.”

DISEASE, PRECLINICAL Disease with no signs or symptoms because these have not yet developed. See also inapparent infection.

DISEASE, SUBCLINICAL A condition in which disease is detectable by special tests but does not reveal itself by signs or symptoms.

DISEASE FREQUENCY SURVEY See cross-sectional study; morbidity survey.

DISEASE INTENSITY See force of morbidity.

DISEASE LABEL The identity of the condition from which a patient suffers. It may be the name of a precisely defined disorder identified by a battery of tests, a probability statement based on consideration of what is most likely among several possibilities, or an opinion based on pattern recognition. Use of the word label can convey stigma. See also diagnosis; semiology; sickness “career.”

DISEASE MAPPING A method for displaying spatial distribution of cases of disease, most often used in veterinary epidemiology. Disease maps may display raw numbers or rates (i.e., choroplethic maps). See also geographic information system; medical geography.

DISEASE MODEL Quantitative simulation of the natural history of a disease (incidence, progression, prognosis, etc.) based on epidemiological data. A public health model is population-based and is used in planning and evaluating health services, whereas a clinical model is used in individual patient care.129

DISEASE ODDS RATIO See odds ratio.

DISEASE PROGRESSION BIAS

  1. In studies on the clinical accuracy and validity of diagnostic tests, a bias that occurs if results of the diagnostic test under study and of the reference standard test are not collected on the same patients at the same time, and if spontaneous recovery or progression to a more advanced stage of disease takes place. See also QUADAS.
  2. In etiologic studies, biases that occur when disease progression entails metabolic and other pathophysiologic changes that alter the characteristics or concentrations (e.g., in blood, adipose tissue, target organs, peritumoral tissue) of the study exposure biomarkers. Biomarkers of exposure collected during subclinical or overt disease will then not reflect exposures of etiologic significance that took place in more distant time windows. For instance, during the progression of some cancers, blood concentrations of lipophilic substances of putative etiologic interest (e.g., lipophilic vitamins, organochlorine compounds) may be increased or decreased due to pathophysiologic changes associated with cancer-induced weight loss, cholestasis or lipid mobilization.

DISEASE REGISTRY See register, registry.

DISEASE TAXONOMY See taxonomy of disease.

DISEASES OF COMPLEX ETIOLOGY Diseases that result from complex causal pathways (e.g., from interactions between sociocultural, environmental, clinical, genetic, and epigenetic processes) often over long periods of life. Common diseases with late-onset phenotypes often result from interactions between the epigenome, the genome, and the environment.43,116 See also polygenic diseases; web of causation.

DISENTRAPMENT Some form of escape from demographic entrapment.

DISINFECTION Killing of infectious agents outside the body by direct exposure to chemical or physical agents.

Concurrent disinfection is the application of disinfective measures as soon as possible after the discharge of infectious material from the body of an infected person or after the soiling of articles with such infectious discharges, all personal contact with such discharges or articles being minimized prior to such disinfection.

Terminal disinfection is the application of disinfective measures after the patient has been removed by death or to a hospital, or has ceased to be a source of infection, or after other hospital isolation practices have been discontinued. Terminal disinfection is rarely practiced; terminal cleaning generally suffices, along with airing and sunning of rooms, furniture, and bedding. Disinfection is necessary only for diseases spread by indirect contact; steam sterilization or incineration of bedding and other items is desirable after a disease such as plague or anthrax.52

DISINFESTATION Any physical or chemical process serving to destroy or remove unde- sired small animal forms, particularly arthropods or rodents, present upon the person, the clothing, or in the environment of an individual or on domestic animals. Disinfes- tation includes delousing for infestation with Pediculus humanus humanus, the body louse. Synonyms include the terms disinsection and disinsectization when insects only are involved.

DISMOD A model designed to help disease experts arrive at internally consistent estimates of incidence, duration, and case fatality rates of burden of disease. It uses a life-table approach in following an initially disease-free cohort over time while applying the risks (incidence, remission, case fatality rate) associated with a disease and the com- peting risk of all other diseases as represented by general mortality. Using a competing risk approach, it calculates the relative proportions of each cohort that will develop, recover from, or die from the disease, die from other causes of mortality, or continue to live disease-free. To the extent that it takes into account other competing risk factors of mortality, this approach may be more realistic than one that simply assumes that Prevalence = incidence x duration. Differences between the two approaches seem most marked in chronic diseases with low rates of remission and cause-specific mortality.46

DISORDER, DISEASE, SYNDROME A disorder is a disturbance or departure—for example, of an organ or body system—from normal healthy function; i.e., an impair- ment. A disease is a disorder that can be assigned to a diagnostic category; it usually has a distinct clinical course and often a distinct etiology. A syndrome is a group of symptoms and signs that tend to appear together and collectively characterize a disorder.

DISTAL DETERMINANT See determinant, distal (distant).

DISTRIBUTION

  1. (Syn: frequency distribution) The complete summary of the frequencies of the values or categories of a measurement made on a group of persons. The distribution tells either how many or what proportion of the group was found to have each value (or each range of values) out of all the possible values that the measurement can have.
  2. In statistics, a distribution function.

DISTRIBUTION-FREE METHOD (Syn: nonparametric method) A method that does not depend upon the form of the underlying distribution. Like all statistical methods, however, it depends on assumptions of randomness (in sampling or in allocation to exposure) and thus may be subject to bias.

DISTRIBUTION FUNCTION (Syn: probability distribution) A mathematical function that gives the relative frequency or probability with which a random variable falls at or below each of a series of values. Examples include the normal distribution, log-normal distribution, chi-square distribution, t distribution, F distribution, logistic distribution, binomial distribution, Poisson distribution, and multinomial distribution, all of which have applications in epidemiology.

DMF Decayed, missing, and filled teeth. Lowercase letters (i.e., dmf) are used for deciduous dentition, uppercase for permanent teeth. The DMF number is widely used in dental epidemiology.

DOMINANCE A concept from biological and urban ecology describing how one group or species has more influence or control than the others. In an urban community, dominance may be related to competition over land value, strategic geographical location, or a healthier environment.18

DOMINANT In genetics, alleles that fully manifest their phenotype when present in the heterozygous state. Contrast recessive.

DOSE The amount of a substance available for interaction with metabolic processes or biologically significant receptors after crossing the relevant boundary (epidermis, gut, respiratory tract); the absorbed dose is the amount crossing a specific absorption bar- rier. In lay usage, the amount of a medication to be taken at one time.130

DOSE-RESPONSE RELATIONSHIP (Syn: dose-effect relationship) An association between a given dose or set of doses (i.e., amount, duration, concentration) of an agent and the magnitude of a graded effect in an individual or a population.The relationship of observed outcomes (responses) in a population to varying levels of a protective or harmful agent such as a drug or an environmental contaminant. Commonly displayed as a graph, sometimes as a histogram. Important aspects include the rate at zero dose (base- line, or control), the presence or absence of a threshold dose, the presence or absence of monotonicity, and the form of the mathematical expression that better fits the rela- tionship between the dose and the response (linear, logarithmic, etc.). Other relevant fea- tures include the time element (e.g., How soon after the dose is the response observed? Is there a latent period?) and the range of individual variation (What proportion of those exposed experience no response, and a slight, moderate, or severe response?).

DOT CHART, DOT PLOT A display (plot) of the individual values of a set of numbers. The x axis represents categories of a noncontinuous variable and the y axis represents the values displayed by the observations.

DOUBLE-BLIND TRIAL A procedure of blind assignment to study and control groups and blind assessment of outcome, designed to ensure that ascertainment of outcome is not biased by knowledge of the group to which an individual was assigned. Double refers to both parties—the observer(s) in contact with the subjects and the subjects in the study and control groups. See also blind(ed) study; randomized controlled trial.


DOUBLING TIME The average time taken for a population to double in numbers.

DRIFT See genetic drift; social drift.

DROPLET NUCLEI A type of particle implicated in the spread of airborne infection. Droplet nuclei are tiny particles (1–10 μm diameter) that represent the dried residue of droplets. They may be formed by (1) evaporation of droplets coughed or sneezed into the air or (2) aerosolization of infective materials. See also transmission of infection.

DROPOUT A person enrolled in a study who becomes inaccessible or ineligible for fol- low-up (e.g., because of inability or unwillingness to remain enrolled in the study). The occurrence of dropouts can lead to biases in study results. See also attrition bias.

DRUG RESISTANCE The ability of an organism to develop strains that are impervious to specific threats to their existence.131

DRUG RESISTANCE, MULTIPLE (MDR) Simultaneous resistances to several structur- ally and functionally distinct drugs.

DRUG-RESISTANT TUBERCULOSIS, MULTIPLE (MDR tuberculosis) A form of tuber- culosis that is resistant to two or more of the primary drugs used for the treatment of tuberculosis (at least isoniazid and rifampin). It occurs when bacteria develop the ability to withstand antibiotic attack and relay that ability to newly produced bacteria. It can spread from one person to another. On an individual basis, improper use of the antituberculosis medications remains an important cause of drug-resistant tuberculosis.132 Treatment of MDR tuberculosis requires second-line drugs (SLDs) that are less effective, more toxic, and costlier than first-line isoniazid- and rifampin-based regimens.

DRUG-RESISTANT TUBERCULOSIS, EXTENSIVELY (XDR) A form of tuberculosis caused by a strain of Mycobacterium tuberculosis resistant to isoniazid and rifampin, to any fluoroquinolone, and to at least one of the three following injectable drugs: capreo- mycin, kanamycin, and amikacin.133

DUMMY VARIABLE See indicator variable.

DYNAMIC POPULATION A population that gains and loses members; all natural populations are dynamic—a fact recognized by the term population dynamics, which is used by demographers to denote changing composition. See also population dynamics; stable population; fixed cohort.

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