TACTICS The detailed directions and instructions designed to achieve an aim or target. See strategy.

TARGET An aspired outcome that is explicitly stated. What a health policy or program will achieve by a specified date; for example, reduced unwanted pregnancy rates, lower teenage smoking rates, enhanced QALYs. Usually but not necessarily expressed in quantitative terms.


  1. The collection of individuals, items, measurements, etc., about which inferences are desired. The term is sometimes used to indicate the population or group from which a sample or study population is drawn and sometimes to denote a reference population about which inferences are desired.
  1. The group to which inference from the study is directed.
  1. The group of persons for whom an intervention is planned.

TAXON (plural, taxa) The general term for a group or entity; e.g., a species or family in a taxonomy.

TAXONOMY A systematic classification into related groups.

TAXONOMY OF DISEASE The orderly classification of diseases into appropriate categories on the basis of relationships among them, with the application of names. See also nosography, nosology.

TCDD 2,3,7,8-tetrachlorodibenzo-p-dioxin. The strongest agonist of the arylhydrocarbon receptor (AhR). A nongenotoxic human carcinogen.219 A by-product of the manu- facture of polychlorinated phenols that is generated through waste incineration. Both epidemiological evidence and mechanistic studies have indicated a relationship between TCDD exposure and the occurrence of some cancers. Many xenobiotic metabolizing enzymes are induced through TCDD-mediated AhR activity. To assess the clinical and epidemiological impact of compounds like dioxins and other peristent organic pollutants (POPs) is one of the challenges faced by molecular epidemiology, environ- mental epidemiology, and related approaches. See also biological monitoring.

T-DISTRIBUTION, T-TEST The t-distribution is the distribution of a quotient of inde- pendent random variables, the numerator of which is a standardized normal variate and the denominator of which is the positive square root of the quotient of a chi-square distributed variate and its number of degrees of freedom. The t-test uses a statistic that, under the null hypothesis, has the t-distribution to test whether two means differ signifi- cantly, or to test linear regression or correlation coefficients. The t-distribution and the t-test were developed by W. S. Gossett, who wrote under the pseudonym “Student,” as his employment precluded individual publication.

TEPHINET Training Programs in Epidemiology and Public Health Interventions Network (www.tephinet.org). A professional alliance of field epidemiology training programs (FETPs) located in some 30 countries around the world.

TERATOGEN A substance that produces abnormalities in the embryo or fetus by disturbing maternal homeostasis or acting directly on the fetus in utero.

TEST OF SIGNIFICANCE See P value; significance, statistical.

TEST HYPOTHESIS In statistics, the hypothesis subject to a statistical test, such as a significance test. A test hypothesis may concern any possible value for the measure under study; e.g., it test whether a risk ratio is 0.5, 1, 2, 4, or any other value of interest. The null hypothesis is a special case of test hypothesis.

THE 3 BY 5 TARGET (Syn: the 3 by 5 initiative) A target set by WHO of providing anti- retroviral therapy (ART) to 3 million people in resource-limited countries by the end of the year 2005. http://www.who.int/3by5/en.


  1. The discipline of how to study the occurrence of phenomena of interest in the health field;5 epidemiological methodology.
  1. The development of mathematical and statistical models to explain different aspects of the occurrence of a variety of diseases. With some infectious diseases, for instance, models have been generated to elucidate the reasons for epidemics and to predict the behavior of the disease as a reaction to given control measures. See also mechanistic epidemiology; model.

THERAPEUTIC TRIAL See clinical trial; randomized controlled trial.

THRESHOLD DOSE The dose above which effects occur.

THRESHOLD LIMIT VALUE See safety standards.

THRESHOLD PHENOMENA Events or changes that occur only after a certain level of a characteristic is reached.

THRIFTY PHENOTYPE A term coined by D. J. P. Barker to describe the hypothesis that insulin resistance and type 2 diabetes originate through undernutrition in the womb.

THRIFTY PHENOTYPE HYPOTHESIS The hypothesis proposing that an undernour- ished baby becomes thrifty. It maintains high levels of sugar in the bloodstream to ben- efit the brain but less sugar in muscles. Muscle growth may be ‘‘traded off’’ to protect the brain. Once adopted, this thrifty behavior becomes permanent and, combined with adiposity in later life, leads to type 2 diabetes. See also “brain sparing”; developmental origins hypothesis.

TIME CLUSTER See clustering.

TIME ORDER The only necessary property of cause: a cause must always precede an effect. Conceptually obvious as it may seem, assessment of the temporal relationship relies heavily on an often complex and subtle assessment of tests and designs.

TIME-PLACE CLUSTER See clustering.

TIME SERIES A single-group research design in which measurements are made at sev- eral different times, thereby allowing trends to be detected. An interrupted time series features several measurements both before and after an intervention and is usually more valid than a simple pretest-posttest design. A multiple time series involves several groups, including a control group.

TIME TRADE-OFF A method of determining utility in which members of a panel express preferences either for normal life expectancy in a defined suboptimal health state or reduced life expectancy in good health.379 The magnitude of reduced life expectancy is varied until there is equipoise between the choices.

“TIME WILL TELL” BIAS A form of interpretive bias that occurs because different scien- tists need different amounts of confirmatory evidence. Certainly the position that more evidence is necessary before making a judgment indicates a judicious attitude that is central to scientific scepticism.34 See also consistency; precautionary principle; replication.

TOLERANCE In toxicology and pharmacology, the adaptive state characterized by dimin- ished effects of a particular dose of a substance.

TORT A legal term for the harmful consequence of an act. Such acts are tried in courts of law, and damages are awarded if wrong or harm is demonstrated. A “toxic tort” is a lawsuit centered around a claim for harm due to a toxic chemical. Epidemiologists sometimes have to testify in legal cases involving tort.

TOTAL FERTILITY RATE (TFR) The average number of children that would be born per woman if all women lived to the end of their childbearing years and bore children according to a given set of age-specific fertility rates. It is computed by summing the age-specific fertility rates for all ages and multiplying by the interval into which the ages are grouped. The TFR is an important fertility measure, providing the most accurate answer to the question “How many children does a woman have on average?”

TOWNSEND SCORE An index of social deprivation developed by the British social sci- entist Peter Townsend (b.1928), used mainly in the United Kingdom; based on numbers economically active but unemployed, households with no car, households not owner- occupied, households overcrowded. The Townsend score uses readily available census data and can be used to rank administratively defined jurisdictions. See also Jarman score; overcrowding.380

TOXICOLOGY The scientific discipline involving the study of actual or potential danger presented by the harmful effects of chemicals (poisons) on living organisms and eco- systems; of the relationship of such harmful effects to exposure; and of the mechanisms of action, diagnosis, prevention, and treatment of intoxications.200 Toxicology has an increasingly broad interface with epidemiology.

TRACER DISEASE METHOD Tracer or indicator conditions are easily diagnosed, rea- sonably frequent illnesses or health states whose outcomes are believed to be affected by health care and that, taken in aggregate, should reflect the gamut of patients and health problems encountered in a medical practice.381 The extent to which the recorded care of these conditions concurs with preset standards of care is used as an index of the quality of care delivered. However, it should first be shown that the preset standards contribute to a favorable outcome. See also sentinel health event.

TRANSCRIPTION Copying of a strand of DNA to generate a complementary strand of RNA.

TRANSDISCIPLINARITY The philosophical concept of scholarly inquiry that ignores con- ventional boundaries among ways of thinking about and solving problems.382 It is based on recognition of the complexity of many problems confronting humans and seeks to mobilize all pertinent scholarly disciplines: physical, biological, social and behavioral sciences, ethics, moral philosophy, communication sciences, economics, politics, and the humanities. Many problems in public health have required transdisciplinary analyses and solutions. Epidemiology thus has a long tradition of transdisciplinary analysis of complex problems, of integrative research (i.e., research that integrates multiple dis- ciplines and levels of analysis), and of multidimensional intervention on the popula- tion and individual determinants on health. The ecological impact of global climate change—which includes the epidemiological impact—is a most current and relevant example of the important contributions of epidemiology to transdisciplinary research.138 Sometimes it may be practically opposed to—but it is not an antonym of—reductionism.

TRANSMISSION OF INFECTION Transmission of infectious agents. Any mechanism by which an infectious agent is spread from a source or reservoir to another person. These mechanisms are defined as follows:

  1. Directtransmission.Directandessentiallyimmediatetransferofinfectiousagentstoa
    receptive portal of entry through which human or animal infection may take place. This may be by direct contact such as touching, kissing, biting, or sexual intercourse or by the direct projection (droplet spread) of droplet spray onto the conjunctiva or the mucous membranes of the eyes, nose, or mouth. It may also be by direct exposure of susceptible tissue to an agent in soil, compost, or decaying vegetable matter or by the bite of a rabid animal. Transplacental transmission is another form of direct transmission.
  1. Indirect transmission. Vehicle-borne: Contaminated inanimate material or objects (fomites) such as toys, handkerchiefs, soiled clothes, bedding, cooking or eating utensils, and surgical instruments or dressings (indirect contact); water, food, milk; biological products including blood, serum, plasma, tissues, or organs; or any substance serving as an intermediate means by which an infectious agent is transported and introduced into a susceptible host through a suitable portal of entry. The agent may or may not have multiplied or developed in or on the vehicle before being transmitted. Vector-borne: (a) Mechanical: Includes simple mechanical carriage by a crawling or flying insect through soiling of its feet or proboscis or by passage of organisms through its gastrointestinal tract. This does not require multiplication or development of the organism. (b) Biological: Propagation (multiplication), cyclic development, or a combination of these (cyclopropagative) is required before the arthropod can transmit the infective form of the agent to humans. An incubation period (extrinsic) is required following infection before the arthropod becomes infective. The infectious agent may be passed vertically to succeeding generations (transovarian transmission); transstadial transmission is its passage from one stage of the life cycle to another, as nymph to adult. Transmission may be by saliva during biting or by regurgitation or deposition on the skin of feces or other material capable of penetrating subsequently through the bite wound or through an area of trauma from scratching or rubbing. Transmission by an infected nonvertebrate host must be differentiated for epidemiological purposes from simple mechanical carriage by a vector in the role of a vehicle. An arthropod in either role is termed a vector.

Airborne: The dissemination of microbial aerosols to a portal of entry, usually the respiratory tract. Microbial aerosols are suspensions in the air of particles consisting partially or wholly of microorganisms. Particles in the range of 1 to 5 μm are easily drawn into the alveoli of the lungs and may be retained there. They may remain suspended in the air for long periods.

Airborne transmission includes:
Droplet nuclei: residues that result from evaporation of fluid from droplets emitted by an infected host. Droplet nuclei also may be created purposely by a variety of atomizing devices, or accidentally, as in microbiology laboratories or in abattoirs, rendering plants, or autopsy rooms. They usually remain suspended in the air for long periods.
Dust: The small particles of widely varying size that may arise from soil (fungus spores) or from clothes, bedding, or contaminated floors.
See also acquaintance network; airborne infection; carrier; common vehicle spread; contact; contamination; droplet nuclei.

TRANSMISSION PARAMETER (r) In infectious disease epidemiology, the propor- tion of total possible contacts between infectious cases and susceptibles that lead to new infections.

TRANSOVARIAL TRANSMISSION See vector-borne infection.

TRANSPORT HOST See paratenic host.

TRANSVECTION An epigenetic phenomenon that results from an interaction between an allele on one chromosome and the corresponding allele on the homologous chromosome. It can lead to either gene activation or repression. See also epigenetic inheritance.

TRAP, DEMOGRAPHIC See demographic entrapment.

TREND A long-term movement in an ordered series (e.g., a time series). An essential feature is that the movement, while possibly irregular in the short term, shows movement consistently in the same direction over a long term. Also used loosely to refer to an asso- ciation that is consistent in several samples or strata but is not statistically significant.

TREND (STATEMENT) A proposal for an structured approach to reporting and evalu- ating studies of behavioral and public health interventions that use nonrandomized designs.95,383 See also consort; quadas, quorom; stard; strobe.

TREND LINE The line that best fits the distribution of a set of values plotted on two axes.

TRIAGE The process of selecting for care or treatment those of highest priority or, when resources are limited, those thought most likely to benefit. From the French trier, to separate, choose. See also rapid epidemiological assessment.

TRIAL See clinical trial.

TRIAL PROFILE See consort.

“TRIMMING” (data trimming) The practice, which can be akin to a form of scientific fraud or misrepresentation, of excluding from analysis observations or measurements that lie outside the range the investigator expects; the grounds for exclusion are that these outlying observations would distort the results. Data trimming is permissible only when rules written in advance in the research protocol specify circumstances in which it may be done. Even then it should be done with caution, and openly. See also outliers.

TRIPLE BLIND STUDY A study in which subjects, observers, and analysts are blinded as to which subjects received what interventions. See also blind(ed) study.

TROHOC STUDY A retrospective case-control study. The term, proposed by A. R. Feinstein,322 is the inversion of “cohort”; its use is rare.

TRUE-POSITIVE RATE See sensitivity and specificity.

TRUE-NEGATIVE RATE See sensitivity and specificity.

TUBERCULOSIS A chronic disease since Neolithic times, afflicting an estimated 0.2% of the world’s population, caused by Mycobacterium tuberculosis.384 It merits mention in this dictionary because it continues to present an epidemiological challenge.385 The tuberculin skin test, which has long been a simple, cheap means of screening, is less effi- cient in populations that have been vaccinated with bacille Calmette-Guérin (BCG) and in which there are large numbers of immunocompromised persons infected with HIV.

TUKEY’S METHOD See multiple comparison techniques.

TWIN STUDY Method of detecting genetic etiology in human disease. The basic premise of twin studies is that monozygotic twins, being formed by the division of a single fertilized ovum, carry identical genes, while dizygotic twins, being formed by the fertilization of two ova by two different spermatozoa, are genetically no more similar than two siblings born after separate pregnancies.

TWO-BY-TWO TABLE See two-way table.

TWO-TAIL TEST A statistical significance test based on the assumption that deviation from the test hypothesis in either direction is of interest. See also one-tail test.

TWO-WAY TABLE A contingency table for categorical variables. A table with r rows and c columns in which the entry in each cell represents the frequency for each outcome. Such a table is called an r-by-c table (e.g., a 3-by-4 table). If r = 2 and c = 2 then it is called a two-by-two table.

TYPE I ERROR See error, type i.

TYPE II ERROR See error, type ii.

TYPE III ERROR See error, type iii.

“TYPHOID MARY” A slang expression for an individual who unwittingly transmits infection to others. The original Typhoid Mary, Mary Mallon, was an itinerant cook and an infamous carrier of typhoid in New York City and environs early in the twentieth century.