Content validity: judging the relevance, comprehensiveness, and comprehensibility of an outcome measurement instrument – a COSMIN perspective

Content validity is key when developing a new outcome measurement instrument or selecting an existing measurement instrument. Without good content validity, wrong conclusions can be drawn about an outcome that is measured. In this article, we explain that content validity refers to the relevance, comprehensiveness, and comprehensibility of the instruments' content. Content validity is not only relevant for patient-reported outcome measures but also for other outcome measurement instruments, such as clinician-reported outcome measures and performance-based tests. It is preferably assessed in qualitative studies, eg, focus group studies or Delphi studies. When developing an instrument, the focus is first on the relevance and comprehensiveness of the content of the instrument, then the content is developed, and next its comprehensibility is evaluated. Steps may be repeated. When evaluating content validity of an existing measurement instrument, the three aspects of content validity are evaluated at the same time. Content validity helps to understand whether the content of the measurement instrument is a sufficient representation of the outcome being measured (relevance and comprehensiveness), and whether the formulation used in the measurement instrument (eg, to describe instructions, items, tasks, explanations) is sufficiently understood (comprehensibility) by the people involved in the measurement.