F1 (“F one”) Term used in genetics to describe first-generation progeny of a mating.

F TEST Most commonly, a test used to test that several groups come from populations with the same variance.

FACE VALIDITY The extent to which a measurement or a measurement instrument appears reasonable on superficial inspection. See also validity and validity, measurement.


  1. An event, characteristic, or other definable entity that leads to a change in a health condition or other defined outcome. See also causality; causation of disease, factors in.
  2. A synonym for (categorical) independent variable, or, more precisely, an independent variable used to identify, with numerical codes, membership of qualitatively different groups. A causal role may be implied, as in “overcrowding is a factor in disease transmission,” where overcrowding represents the highest level of the factor “crowding.”

FACTOR ANALYSIS A set of statistical methods for analyzing the correlations among several variables in order to estimate the number of fundamental dimensions that underlie the observed data and to describe and measure those dimensions. Used fre- quently in the development of scoring systems for rating scales and questionnaires.

FACTORIAL DESIGN A method of setting up an experiment or study to ensure that all levels of each intervention or classificatory factor occur with all levels of the others.

FALSE NEGATIVE Negative test result in a person who possesses the attribute for which the test is conducted. The labeling of a diseased person as healthy when screening in the detection of disease. See also screening; sensitivity.

FALSE POSITIVE Positive test result in a person who does not possess the attribute for which the test is conducted. The labeling of a healthy person as diseased when screening in the detection of disease. See also screening; specificity.

FAMILIAL AGGREGATION A tendency of some diseases to cluster in families, which may be the result of a genetic mechanism, an environmental factor or process common to family members (e.g., diet), or a combination of both. Ascertainment bias should be seriously considered.

FAMILIAL DISEASE Disease that exhibits a tendency toward familial occurrence or aggregation. Familial occurrence of disease may be due to genetic transmission, intrafamilial transmission of infection or culture, interaction within the family, or the family’s shared experience, including its exposure to common environmental factors.23,134

FAMILY A group of two or more persons united by blood, adoptive or marital ties, or the common-law equivalent; the family may include members who do not share the house- hold but are united to other members by blood, adoptive or marital ties, or equivalent ties. Epidemiological studies may be concerned with family members or with those who share the same household or dwelling unit.

FAMILY, EXTENDED A group of persons comprising members of several generations united by blood, adoptive and marital ties, or equivalent ties. See also family, nuclear.

FAMILY, NUCLEAR A group of persons comprising members of a single or at most two generations, usually husband-wife-children, united by blood, adoptive or marital ties, or equivalent ties.

FAMILY CONTACT DISEASE Disease that occurs among members of the family of a worker who is exposed to a toxic substance such as asbestos dust and carries this home on his or her person or clothing, causing exposure to other family members.

FAMILY OF CLASSIFICATIONS The Conference for the Tenth Revision of the International Statistical Classification of Diseases and Related Health Problems recommended adopting the concept of the family of disease and health-related classifications.202 This “family” comprises ICD-10 (the ICD three-character core classification), its short tabulation lists, and the ICD four-character classification; lay reporting and other community-based information schemes in health; specialty- based adaptations for oncology, psychiatry, etc.; other health-related classifications (ICIDH, procedures, reasons for encounter); and the International Nomenclature of Diseases (IND).

FAMILY STUDY An epidemiological study of a family or a group of families. The term has

been used to describe surveillance of family groups (e.g., for tuberculosis). In genet- ics, investigation of families showing an unusual characteristic in order to determine whether the characteristic clusters in certain families, and if so, why.

FARR’S LAWS OF EPIDEMICS William Farr (1807–1883), who was the first Com- piler of Abstracts in the General Register Office of England and Wales, enunciated several “laws” of epidemics.203 He observed that epidemics appear to be generated in unhealthy places, go through a regular course, and decline. In his Second Annual Report (1840), he demonstrated mathematically that the decline in mortality of a waning epidemic occurs at a uniformly accelerating rate. He constructed mathe- matical models to explain the natural history of epidemic diseases, often correctly and elegantly.

FATALITY RATE The death rate observed in a designated series of persons affected by a simultaneous event (e.g., victims of a disaster). A term to be avoided, because it can be confused with case fatality rate.

F DISTRIBUTION (Syn: variance ratio distribution) The distribution of the ratio of two independent quantities each of which is distributed like a variance in normally distrib- uted samples. So named in honor of R. A. Fisher (1890–1962), who first described this distribution.

FEASIBILITY (of a study, program, intervention) The viability, practicability, or workabil- ity of the study, program or intervention. How possible or practicable it is to carry it out. The feasibility of clinical and epidemiological research studies is strongly influenced by clinical, cultural, logistic, economic, and ethical factors. In assessing the weaknesses of a study in terms of internal and external validity and statistical precision, the feasibility of the theoretically better alternatives must be considered.

FEASIBILITY STUDY Preliminary study to determine the practicability of a proposed health program or procedure or of a larger study and to appraise the factors that may influence its practicability. See also pilot investigation, study.

FECUNDITY The ability to produce live offspring. Fecundity is difficult to measure, since it refers to the theoretical ability of a woman to conceive and carry a fetus to term. If a woman produces a live birth, it is known that she and her consort were fecund during some time in the past.

FEMALE-MALE GAP A set of national, regional, or other estimates—for example, of health status or literacy—in which all the figures for females are expressed as a percent- age of the corresponding figures for males, which are indexed to 100. In some countries, a useful indicator of selective female abortion and infanticide.

FERTILITY The actual production of live offspring. Stillbirths, fetal deaths, and abortions are not included in the measurement of fertility in a population. See also gravidity; parity.

FIELD EPIDEMIOLOGY The practice of epidemiology in the field—in the community— commonly in a public health service (i.e., a unit of government or a closely allied institu- tion). Field epidemiology is how epidemics and outbreaks are investigated, and it is a tool for implementing measures to protect and improve the health of the public. Field epidemiologists must deal with unexpected, sometimes urgent problems that demand immediate solution. Its methods are designed to answer specific epidemio- logical questions in order to plan, implement, and/or evaluate public health inter- ventions. These studies must consider the needs of those who will use the results. The task of a field epidemiologist is not complete until results of a study have been clearly communicated in a timely manner to those who need to know and an inter- vention has been made to improve the health of the people.204 See also applied epidemiology.

FIELD SURVEY The planned collection of data in “the field,” usually among noninstitu- tionalized persons in the general population.27 A method of establishing a relationship between two or more variables in a population in numerical terms by eliciting and col- lating information from existing sources (not only records but people who can say how they feel or what happened). See also cross-sectional study.

FIELD TRIALS Clinical trials or other types of clinical studies (e.g., on vaccines, drugs, bed nets, exercise programs) conducted outside the laboratory, in the general population, in primary care; often, as opposed to studies in academic, tertiary care settings. See also pragmatic study.

FISHER’S EXACT TEST The test for association in a two-by-two table that is based upon the exact hypergeometric distribution of the frequencies within the table.

FISHING EXPEDITION Exploratory study to find clues and leads for further study. Although the term is sometimes used pejoratively, such “expeditions” may be done for worthwhile causes (e.g., to seek clues to the cause of a major life-threatening outbreak). See also data dredging.

FITNESS This word has specific meanings in several fields related to epidemiology.
1. In population genetics, a measure of the relative survival and reproductive success of a given phenotype or population subgroup.
2. In health promotion and health risk appraisal, physical fitness is a set of attributes

that people have or achieve that relate to their ability to perform physical activity. Intellectual and emotional fitness can also be described and to some extent measured.

FIXED COHORT A cohort in which no additional membership is allowed—that is, it is fixed by being present at some defining event (“zero time”); an example is the cohort comprising survivors of the atomic bomb exploded at Hiroshima. See also dynamic population.

FLOW CHART See flow diagram.

FLOW DIAGRAM (Syn: logic model, flow chart) A diagram comprising blocks connected by arrows representing steps in a process. An algorithm used in decision analysis. Flow diagrams have many uses (e.g., to show eligibility, recruitment, and losses in design and execution of a study or to show how a program is intended to work).

FOCUS GROUP A small convenience sample of people brought together to discuss a topic or issue with the aim of ascertaining the range and intensity of their views rather than arriving at a consensus. This sociological method is used by epidemiologists to, for example, appraise perceptions of health problems, assess acceptability of a field study, or refine the questions to be used in a field study. The distinction between a focus group and a delphi survey is that the latter does aim to reach a consensus, is more formal, is usually made up of experts, generally functions by mail or telephone, and the identities of members preferably are unknown to one another. In a focus group, persons meet face to face, although it is possible for their identities to remain unknown to one another.

FOCUS OF INFECTION A defined and circumscribed locality containing the epidemiological factors needed for transmission: a human community, a source of infection, a vector population, and appropriate environmental conditions.

FOLLOW-UP Observation over a period of time of an individual, group, or an initially defined population whose appropriate characteristics have been assessed in order to observe changes in health status or health-related variables. See also attrition bias; cohort.


1. A study in which individuals or populations—selected on the basis of whether they have been exposed to risk, received a specified preventive or therapeutic procedure, or possess a certain characteristic—are followed to assess the outcome of exposure, the procedure, or effect of the characteristic (e.g., occurrence of disease).

2. Synonym for cohort study.

FOMITES (singular, fomes) Articles that convey infection to others because they have been contaminated by pathogenic organisms. Examples include a handkerchief, drinking glass, door handle, clothing, and toys.

FORCE OF MORTALITY [Syn: actuarial (death) rate] The hazard rate of the occurrence of death at a point in time t—the limit as ∆ t approaches zero—of the probability that an individual alive at time t will die by time t + ∆ t, divided by ∆ t. Distinct from cumula- tive death rate.

FORECASTING A method of estimating what may happen in the future that relies on extrapolation of existing trends (demographic, epidemiological, etc.). It may be less use- ful than scenario building, which has greater flexibility. For example, extrapolation of mortality trends for coronary heart disease in the early 1960s in the United States sug- gested that the mortality rates would continue to rise, whereas in fact the rates began to fall soon after that time.

FORENSIC EPIDEMIOLOGY The use of epidemiological reasoning, knowledge, and methods in the investigation of public health problems that may have been caused by or associated with intentional and/or criminal acts.205 As mentioned above, epidemiology is applied in many populations, professional specialties, and health care settings; hence this dictionary includes just some examples of definitions for a few specialty-based branches of epidemiology.

FOREST PLOT A plot that summarizes results of studies included in a meta-analysis. The name was introduced at a time when the graph had vertical lines representing the studies.

FORTUITOUS RELATIONSHIP See association, fortuitous.

FORWARD SURVIVAL ESTIMATE A procedure for estimating the age distribution at some later date by projecting forward an observed age distribution. The procedure uses survival ratios, often obtained from model life tables.

FOURFOLD TABLE See contingency table.

“FOURTH WORLD” The environmental and socioeconomic situation of decayed urban neighborhoods in affluent nations, resembling the conditions encountered in the poorest developing countries. It includes homeless people, who are among an underclass (often disenfranchised) found in urban communities in rich countries. This term should not be used without explanation in scientific writing.

FRACTALS Mathematical patterns in which smaller parts have the same shape as larger parts, indefinitely down to ever finer levels of magnification. Blood vessels and the bronchial tree behave according to fractal theory. An application of fractals occurs in studies of the way human and other populations grow and spread. The same rules may apply to the spread of some infections and neoplasms.

FRAGILE DATA Data derived from a well-designed study that do not quite reach a level of statistical significance but arrive at unexpected and/or important conclusions. Alternatively, data that reach or imply important conclusions from a poorly designed study. Hence an ambiguous and potentially misleading term.

FRAMINGHAM STUDY Probably the best-known cohort study of heart disease. Since 1949, samples of residents of Framingham, Massachusetts, have been subjects of investigations of risk factors in relation to the occurrence of heart disease and later, other outcomes.

FREQUENCY See occurrence.



FREQUENCY POLYGON A graphic illustration of a distribution, made by joining a set of points, for each of which the abscissa is the midpoint of the class and the ordinate, or height, is the frequency.

FUNCTION A quality, trait, or fact that is so related to another as to be dependent upon and to vary with this other.

FUNDING BIAS Bias in characterizing an association (usually between an exposure and a set of outcomes) owing to the absence or withdrawal of financial and other types of support. Lack of support obstructs or discourages the conduct of research. It may lead to publication bias. See also conflict of interest.

FUNNEL PLOT A plotting device used in meta-analysis to detect publication bias. The estimate of risk is plotted against sample size. If there is no publication bias, the plot is funnel-shaped; but if studies showing positive results are more likely to be published than null studies, there is a “hole in the lower left corner” of the funnel.106, 206