AlphaMED Resources · https://alphamedicine.uk/Resources/glossary/validity-reliability

Validity vs reliability

Validity is whether a test measures what it claims to measure. Reliability is whether it gives consistent results when repeated.

Updated 19 October 2026 (1 minute read)

Summary

  • Validity asks whether an instrument is measuring what it is intended to measure.
  • Reliability asks whether scores differ because people differ, rather than because of measurement error. A satisfactory instrument must be able to distinguish between individuals despite that error.
  • A measure can be reliable and still not be valid. A kitchen scale that always reads 2 kg too heavy gives the same wrong answer every time.
  • A valid measure is usually reliable, but a reliable measure is not automatically valid. Consistency is needed for validity but does not prove it.

Interview questions

  • What is the difference between validity and reliability?
  • You step on the same bathroom scale three times and get three different weights. What does that tell you about the scale?
  • Is a test that gives the same result every time a good test?

Sources

  1. cochrane.orgcochrane.org/…/chapter-18

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