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.

Confidence interval.

A range of values estimated for a given variable. The range has a specified probability, e.g., 95 percent, of including the true value of the variable. The specified probability is called the confidence level.

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 nonrandom, as in an observational cohort study or a case-control study.

EAE.

See Experimental allergic encephalomyelitis.

EAN.

See Experimental allergic neuritis.

Ecologic study.

A study in which the units of analysis are populations or groups of people, rather than individuals.

Encephalitis.

Refers to an encephalopathy caused by an inflammatory response in the brain. This is usually manifested with systemic constitutional symptoms, particularly fever and pleocytosis of the cerebrospinal fluid. However, the terms encephalopathy and encephalitis have been used imprecisely and even interchangeably in the literature.

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. However, the terms encephalopathy and encephalitis have been used imprecisely and even interchangeably in the literature. See Chapter 3.

Erythema multiforme.

An inflammatory eruption characterized by symmetrical erythematous, or edematous lesions of the skin or mucous membranes. It is usually nonpruritic, lasts several days, and leaves no sequelae. Stevens-Johnson syndrome is a rare, severe, potentially fatal form of erythema multiforme that is associated with bullous lesions.

Experimental allergic encephalomyelitis.

Also known as EAE. An experimental model for acute disseminated encephalomyelitis. See Chapter 3.

Experimental allergic neuritis.

Also known as EAN. An experimental model of Guillain-Barré syndrome. See Chapter 3.

GBS.

See Guillain-Barré syndrome.