Source: A Dictionary of Epidemiology (5 ed.) Edited by Miquel Porta. Oxford University Press, 2008. DOI: 10.1093/acref/9780195314496.001.0001

ADJUSTMENT A summarizing procedure for a statistical measure in which the effects of differences in composition of the populations being compared have been minimized by statistical methods. Examples are adjustment by regression analysis and by standardization. Adjustment often is performed on rates or relative risks, commonly because of differing age distributions in populations that are being compared. The mathematical procedure commonly used to adjust rates for age differences is direct or indirect STANDARDIZATION.

ATTACK RATE The cumulative incidence of infection in a group observed over a period during an epidemic. This "rate" can be determined empirically by identifying clinical cases and/or by means of seroepidemiology. Because its time dimension is uncertain or arbitrarily decided, it should probably not be described as a rate. See also INFECTION RATE; MASS ACTION PRINCIPLE; REED-FROST MODEL; SECONDARY ATTACK RATE

CARRIER A person or animal that harbors a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection. The carrier state may occur in an individual with an infection that is inapparent throughout its course (known as healthy or asymptomatic carrier) or during the incubation period, convalescence, and postconvalescence of an individual with a clinically recognizable disease (known as incubatory carrier or convalescent carrier). The carrier state may be of short or long duration (temporary or transient carrier or chronic carrier).1 'Adapted from Chin J, ed. Control of Communicable Disease Manual, 17th ed. Washington, DC: American Public Health Association, 2000.

CASE FATALITY RATE The proportion of cases of a specified condition which are fatal within a specified time.

COMMUNICABLE PERIOD The time during which an infectious agent may be transferred directly or indirectly from an infected person to another person, from an infected animal to humans, or from an infected person to an animal, including ardiropods. See also TRANSMISSION OF INFECTION.

DISEASE, PRECLINICAL Disease with no signs or symptoms because they have not yet developed. See also INAPPARENT INFECTION.

GENERATION TIME The interval between receipt of infection by and maximal infectivity of the host. This applies to both clinical cases and inapparent infections. With person to person transmission of infection, the interval between cases is determined by the generation time. See SERIAL INTERVAL. See also INCUBATION PERIOD.

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

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

INDUCTION PERIOD The period required for a specific cause to produce disease. More precisely, the interval from the causal action of a factor to the initiation of the disease. For example, a span of many years may pass between (presumably) radiation-induced mutations and the appearance of leukemia; this span would be the induction period for radiogenic leukemia. See also CARCINOGENESIS; INCUBATION PERIOD; LATENT PERIOD

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

INFECTION RATE The incidence rate of manifest plus inapparent infections (the latter determined by seroepidemiology). INFECTION TRANSMISSION PARAMETER (r) The proportion of total possible contacts between infectious cases and susceptibles that lead to new infections.

LATENT INFECTION Persistence of an infectious agent within the host without symptoms (and often without demonstrable presence in blood, tissues, or bodily secretions of host).

LATENT PERIOD (Syn: latency) Delay between exposure to a disease-causing agent and the appearance of manifestations of the disease. After exposure to ionizing radiation, for instance, there is a latent period of 5 years, on average, before development of leukemia, and more than 20years before development of certain other malignant conditions. The term latent period is often used synonymously with induction period, that is, the period between exposure to a disease-causing agent and the appearance of manifestations of the disease. It has also been defined as the period from disease initiation to disease detection. In infectious disease epidemiology, this corresponds to the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period). See also INCUBATION PERIOD; INDUCTION PERIOD.

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

MULTIVARIATE ANALYSIS A set of techniques used when the variation in several variables has to be studied simultaneously. In statistics, any analytic method that allows the simultaneous study of two or more DEPENDENT VARIABLES.

NET REPRODUCTIVE RATE (R) (Syn: case reproduction rate) In infectious disease epidemiology, the average number of secondary cases that will occur in a mixed host population of susceptibles and non-susceptibles when one infected individual is introduced. Its relationship to the BASIC REPRODUCTIVE RATE (/^) is given by R=RoX where X is the proportion of the host population that is susceptible.

ODDS RATIO (Syn: cross-product ratio, relative odds) The ratio of two odds. The term odds is defined differently according to the situation under discussion. Consider the following notation for the distribution of a binary exposure and a disease in a population or a sample.
a - exposed with disease
b - unexposed with disease
c - exposed without disease
d - unexposed without disease
The odds ratio (cross-product ratio) is ad/bc. The exposure-odds ratio for a set of case control data is the ratio of the odds in favour of exposure among the cases (a/b) to the odds in favor of exposure among noncases (c/d). This reduces to ad/bc. With incident cases, unbiased subject selection, and a "rare" disease (say,under 2% cumulative incidence rate over the study period), ad/bc is an approximate estimate of the RISK RATIO. With incident cases, unbiased subject selection, and DENSITY SAMPLING of controls, ad/bc is an estimate of the ratio of the person-time incidence rates (FORCE OF MORBIDITY) in the exposed and unexposed (no rarity assumption is required for this). The disease-odds ratio for a cohort or cross sectional study is the ratio of the odds in favor of disease among the exposed (a/c) to the odds in favor of disease among the unexposed (b/d). This reduces to ad/bc and hence is equal to the exposure-odds ratio for the cohort or cross section. The prevalence-odds ratio refers to an odds ratio derived cross-sectionally, as, for example, an odds ratio derived from studies of prevalent (rather than incident) cases. The risk-odds ratio is the ratio of the odds in favor of getting disease, if exposed, to the odds in favor of getting disease if not exposed. The odds ratio derived from a cohort study is an estimate of this.

RATE RATIO (RR) The ratio of two rates. The term is used in epidemiologic research with a precise meaning, i.e., the ratio of the rate in the exposed population to the rate in the unexposed population: RR=Ie/Iu where Ie is the incidence rate among exposed, and Iu is the incidence rate among unexposed. See also RELATIVE RISK.

1. The ratio of the RISK of disease or death among the exposed to the risk among the unexposed; this usage is synonymous with RISK RATIO.
2. Alternatively, the ratio of the cumulative incidence rate in the exposed to the cumulative incidence rate in the unexposed, i.e., the rate ratio.
3. The term relative risk has also been used synonymously with odds ratio and, in some biostatistical articles, has been used for the ratio of FORCES OFMORBIDITY. The use of the term relative risk for several different quantities arises from the fact that for "rare" diseases (e.g., most cancers) all the quantities approximate one another. For common occurrences (e.g., neonatal mortalityin infants under 1500-g birth weight), the approximations do not hold. See also CUMULATIVE INCIDENCE RATIO; ODDS RATIO; RATE RATIO; RISK RATIO

SECONDARY ATTACK RATE The number of cases of an infection that occur among contacts within the incubation period following exposure to a primary case in relation to the total number of exposed contacts; the denominator is restricted to susceptible contacts when these can be determined. The secondary attack rate is a measure of contagiousness and is useful in evaluating control measures. See also ATTACK RATE; BASIC REPRODUCTIVE RATE.

SERIAL INTERVAL (Syn: generation time) The period of time between analogous phases of an infectious illness in successive cases of a chain of infection that is spread person to person.

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