Intra- and inter-observer variation in functional examination of the masticatory system.

Occlusal variables and clinical signs of functional disturbances of the masticatory system were studied with respect to intra- and inter-observer differences in 11-year-old children and adults. Inter-observer differences were present in all registrations made. They were relatively small when measuring the movement capacity of the mandible but large when measuring the distance between RP-IP. The clinical dysfunction index was recorded with an agreement of 82% (Scott's pi = 0.69) in children, 73% (Scott's pi = 0.58) in adults. Repeated registrations performed 5 weeks apart in the child group showed high agreement while repeated registrations with an interval on one year in the adult group revealed considerable variations for mandibular mobility and for the clinical dysfunction index. The study showed that intra-observer constancy is larger than that between different observers. Thus repeated registrations in longitudinal studies should be done by the same observer. In order to control unavoidable observer errors, and to minimize the influence of the large, normal fluctuation of clinical signs that are known to occur, longitudinal studies should include control groups.