Reasons for repeated measurements

There are many reasons for collecting repeated measures data. Some examples are as follows:

• To ensure that a treatment is effective over a specified period of time. Often this will be done using a carefully planned trial with fixed timings for visits.

• To monitor safety aspects of the treatment over a specified period (repeated efficacy measurements are then incidental).

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• To see how long a single dose of a drug takes to become effective by measuring drug concentration or a physiological response at fixed intervals (often over 24 hours). In this situation repeated measurements are taken at a single visit.

• Repeated observations are sometimes inherent in the measurement itself; for example, blood pressure monitors can take measurements as frequently as every 10 seconds.

• To monitorparticular groups of patients over time. Often these are retrospective studies where repeated measurements have been recorded in an unplanned fashion. For example, repeated observations on patients with a particular disease may be available from a hospital clinic.

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