Health Status Measurement, Reliability and Internal Consistency

Reliability refers to the concept of consistency of measurement. Medical decision making is based on observations and measurements taken from the patient. For decision making to be based on the best possible information, such measurements should be both reliable and valid. Reliability of measurement is a necessary condition for a measure to be valid but is not sufficient to ensure validity. A psychometric test, attitude scale, or observational measurement may be reliable in that it can be consistently assessed over time but not valid in that it does not accurately reflect the construct it is designed to measure. In terms of statistical theory, reliability is defined as the ratio of the variation of the true score and the variation of the observed score.

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