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Accuracy: the extent to which a measurement or study result correctly represents the characteristic or relationship that is being assessed.

Acquired immunodeficiency syndrome (AIDS): a disease characterized by a marked reduction in CD4+ T lymphocytes and associated defects in immune response caused by the human immunodeficiency virus (HIV).

Active surveillance: a system of data collection in which those responsible for collecting the information go into the community under observation (typically defined by geographic boundaries) to gather data from various sources.

Acute: a disease of short duration.

Acute myelogenous leukemia (AML): a heterogeneous group of disorders, also known as acute nonlymphocytic leukemia, each of which involves the uncontrolled proliferation of primitive blood-forming cells.

Adjustment: a procedure for overall comparison of two or more populations in which background differences in the distribution of covariables are removed. (See also Standardization.)

Age adjustment: a procedure used to calculate summary rates for different populations in which underlying differences in the age distributions are removed. (See also Age standardization.)

Age-specific rate: a rate (usually incidence or mortality) for a particular age group.

Age standardization (direct): a procedure for obtaining a weighted average of age-specific rates in which the weights are selected on the basis of a standard age distribution (eg, the population of the United States in 1940).

Allele: an alternate form of a gene or a genetic locus that differs from other forms in its specific sequence of nucleotides; certain alleles may affect the structure and function of the corresponding protein coded for by that gene, in turn affecting the susceptibility to a particular condition.

Alpha error: see Type I error.

Alzheimer’s disease: the most common form of dementia in many populations, first described in 1907 by Alois Alzheimer; affected individuals have characteristic abnormalities in their brains, including neurofibrillary tangles and plaques with a protein fragment, β-amyloid, at their core.

Analytic epidemiology: activities related to the identification of possible determinants of disease occurrence.

Analytic study: a research investigation designed to test a hypothesis, often used in reference to a study of an exposure–disease association.

Anthrax: an infectious disease caused by a spore-forming bacterium Bacillus anthracis. The natural occurrence of this illness typically involves inhalation of spores from animal products. This agent was used as a weapon of bioterror in 2001 in the United States, resulting in 22 cases and 5 deaths.

Antibody: a protein, often produced in response to exposure to an antigen, that binds to the antigen and thereby stimulates its inactivation by the immune system.

Antigen: a protein, usually foreign in origin, that is capable of generating an immune response in a host animal.

Antigenic drift: mutation of a pathogen (eg, ...

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