Measuring subjective outcomes: rethinking reliability and validity.

Reliability and validity are criteria used to assess metric adequacy and are typically quantified by correlation coefficients. Reliability is described as the extent to which repeated measurements yield consistent results. Validity is described as the extent to which a measure actually measures what it purports to measure. These conceptualizations are less useful when applied to measures of subjective outcomes because they do not convey other influences that "drive" correlation coefficients. Consistency is a manifestation of a reliable instrument but does not ensure that an instrument is reliable. Establishing the validity of an instrument is a complex process that is heavily dependent on an investigator's hypothesis. Hence, validity coefficients may be more a reflection of hypothesis adequacy than of the extent to which instruments measure what they purport to measure. Appreciating how coefficients are influenced will better enable clinicians to assess the adequacy of subjective outcome measures.