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Table 2 Definitions of domains, measurement properties, and aspects of measurement properties

From: Measurement properties of adult quality-of-life measurement instruments for eczema: protocol for a systematic review

Domain Measurement property Aspect of a measurement property Definition
Reliability    The degree to which the measurement is free from measurement error
Reliability (extended definition)    The extent to which scores for patients who have not changed are the same for repeated measurement under several conditions: for example, using different sets of items from the same HR-PROs (internal consistency) over time (test-retest) by different persons on the same occasion (inter-rater) or by the same persons (that is, raters or responders) on different occasions (intra-rater)
  Internal consistency   The degree of interrelatedness among the items
  Reliability   The proportion of total variance in the measurements which is because of ‘true’a differences among patients
  Measurement error   The systematic and random error of a patient’s score that is not attributed to true change of the construct to be measured
Validity    The degree to which an HR-PRO instrument measures the construct(s) it purports to measure
  Content validity   The degree to which the content of an HR-PRO instrument is an adequate reflection of the construct to be measured
   Face validity The degree to which (the items of) an HR-PRO instrument indeed looks as though they are an adequate reflection of the construct to be measured
  Construct validity   The degree to which the scores of an HR-PRO instrument are consistent with hypotheses (for instance with regard to internal relationships, relationships to scores of other instruments, or differences between relevant groups) based on the assumption that the HR-PRO instrument validly measures the construct to be measured
   Structural validity The degree to which the scores of an HR-PRO instrument are an adequate reflection of the dimensionality of the construct to be measured
   Hypothesis testing Idem construct validity
   Cross-cultural validity The degree to which the performance of the items on a translated or culturally adapted HR-PRO instrument is an adequate reflection of the performance of the items of the original version of the HR-PRO instrument
Responsiveness    The ability of an HR-PRO instrument to detect change over time in the construct to be measured
  Responsiveness   Idem responsiveness
Interpretabilityb    The degree to which one can assign qualitative meaning - that is, clinical or commonly understood connotations - to an instrument’s quantitative scores or changes in scores
  1. Abbreviations: HR-PROs health related patient-reported outcomes, CTT classical test theory. aThe word ‘true’ must be seen in the context of the CTT, which states that any observation is composed of two components - a true score and error associated with the observation. ‘True’ is the average score that would be obtained if the scale were given an infinite number of times. It refers only to the consistency of the score and not to its accuracy [14]. bInterpretability is not considered a measurement property but an important characteristic of a measurement instrument.