psychometric reliabilityThe degree to which a psychometric instrument is free from random error, either by testing the homogeneity of content on multi-item tests with internal consistency evaluation or testing the degree to which the instrument yields stable scores over time.
Reliability pertains to questions concerning whether an instrument is accurate, repeatable, sensitive and is distinguished from validation, which answers whether the instrument—e.g., questionnaire—actually measures the selected "construct" (latent variable). For example, a balance (weight scale) is easily understood as a possibly valid instrument for measuring body weight; its reliability would be assessed by measuring the sensitivity, repeatability and accuracy of the balance. The validity of using the balance for a particular purpose could then be established by comparing the measured reliability to the reliability required for that purpose.
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