ABSTRACT
Repeated measurements arise in many diverse fields, and are possibly even more common than single measurements. The term ‘repeated’ will be used here to describe measurements which are made of the same characteristic on the same observational unit but on more than one occasion. In longitudinal studies individuals may be monitored over a period of time to record the developing pattern of their observed values. Over such a period the conditions may be deliberately changed, as in crossover trials, to study the effects on the individual. Even in studies which are not intentionally longitudinal, once a sample of individual units has been assembled, or identified, it is often easier and more efficient in practice to monitor and observe them repeatedly rather than to discard each one after a single observation and start afresh with another sample.
