The reliability of a measurement or tool is how consistent it is. A reliable measurement or tool will give the same result when repeated at random in the same patient or sample. In clinical trials, reliability is an important consideration in the choice of primary outcome measures (such as an improvement in certain symptoms). The reliability of measures should be formally assessed during the design of clinical trials.

Reliability is different to validity, which is the extent to which a measurement measures what it is supposed to.

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