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Methods sections often describe an instrument as valid and reliable, cite a textbook definition, and provide no supporting evidence. This article explains the two properties and how they differ: validity concerns whether an instrument measures what it is intended to measure, and reliability concerns whether it measures consistently. It describes face, content, construct and criterion validity, distinguishes these from the internal and external validity of a study, and sets out internal consistency, test-retest, inter-rater and parallel-forms reliability.

The article summarizes published guidance on interpreting Cronbach's alpha and the intraclass correlation coefficient, and draws on three doctoral studies, including one in which a scale showed good internal consistency but poor test-retest reliability. It is suited to research-methods and measurement courses at master's and doctoral level. The article is the formal edition of a piece first published by Grad Coach and is released under CC BY 4.0.

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