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without the disease. The relation of an attribute (e.g., immunization) to the disease is examined by comparing the diseased and nondiseased groups with regard to how frequently the attribute is present, or if quantitative, the levels of the attribute, in each of the groups.

Cohort study. (Syn: prospective, follow-up study). A study in which subsets of a defined population can be identified who are, have been, or in the future may be exposed or not exposed, or exposed in different degrees, to a factor or factors hypothesized to influence the probability of occurrence of a given disease (adverse event) or other outcome. The essential feature of the cohort design is observation of the population for a sufficient length of time to generate reliable incidence or mortality rates.

Controlled study. Controlled studies are studies that use a comparison group that differs from the subjects of the study in either disease experience (outcome) or allocation to a regimen (exposure). Allocation to a regimen can be random, as in a randomized clinical trial or study, or nonrandom, as in an observational cohort study or a case-control study.

D

DPT vaccine. See Vaccine, DPT v.

DTP vaccine. See Vaccine, DTP v.

E

Encephalopathy. Refers to a variety of conditions affecting the brain resulting in alterations in the level of consciousness, ranging from stupor to coma. At times, febrile seizures, afebrile seizures, and epilepsy have been considered components of encephalopathy (see Chapter 4).

Erythema multiforme. An inflammatory eruption characterized by symmetrical erythematous, edematous, or bullous lesions of the skin or mucous membranes.

Experimental study. A study in which a population is selected for a planned trial of a regimen (e.g, immunization) whose effects are measured by comparing the outcome of the regimen in the experimental group with the outcome of another regimen in a control group. Allocation of individuals to experimental or control groups is ideally by randomization.

G

Guillain-Barrè syndrome. An acute, usually rapidly progressive form of polyneuropathy characterized by muscular weakness and mild distal sensory loss.

H

Hemolytic anemia. Anemia caused by lysis of red blood cells, which leads