over a long period (commonly years), with comparison of incidence rates in groups that differ in exposure levels. The alternative terms for a cohort study (i.e., follow-up, longitudinal, and prospective study) describe an essential feature of the method, which is observation of the population for a sufficient number of person-years to generate reliable incidence or mortality rates in the population subsets. This generally implies study of a large population, study for a prolonger period (years), or both. The denominators used for analysis may be persons or person-time.4

Complex regional pain syndrome (CRPS): A condition of chronic, severe, and often burning pain usually of part or all of one or more extremities that typically occurs following an injury, that is often accompanied by swelling, skin discoloration, allodynia, abnormal sweating, and impaired motor function in the affected area, and that is of unknown pathogenesis.3

Confounding: Loosely, the distortion of a measure of the effect of an exposure on an outcome caused by the association of the exposure with other factors that influence the occurrence of the outcome. Confounding occurs when all or part of the apparent association between the exposure and outcome is in fact accounted for by other variables that affect the outcome and are not themselves affected by exposure.4

Convulsion: See Seizure.

Crossover experiment: A method of comparing two (or more) treatments or interventions in which subjects, upon completion of one treatment, are switched to the other treatment or intervention. In the case of two treatments, A and B, half the patients are randomly allocated to receive these in the order “A first, then B” and half to receive them in the order “B first, then A.” The outcomes cannot be permanent changes (e.g., they can be symptoms, functional capacity).4

Deltoid bursitis (frozen shoulder): A condition in which joint motion becomes restricted because of inflammatory thickening of the capsule; a common cause of shoulder stiffness.1

Demyelination: Loss of myelin with preservation of the axons or fiber tracts.1

Diphtheria: A specific infectious disease due to the bacterium Corynebacterium diphtheriae and its highly potent toxin; marked by severe inflammation that can form a membranous coating, with formation of a thick fibrinous exudate, of the mucous membrane of the pharynx, the nose, and

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