title: Achieving valid patient-reported outcomes measurement: a lesson from fatigue in multiple sclerosis.
abstract: BACKGROUND
  The increasing influence of patient-reported outcome (PRO) measurement instruments indicates their scrutiny has never been more crucial. Above all, PRO instruments should be valid: shown to assess what they purport to assess.
  
  
  OBJECTIVE
  To evaluate a widely used fatigue PRO instrument, highlight key issues in understanding PRO instrument validity, demonstrate limitations of those approaches and justify notable changes in the validation process.
  
  
  METHODS
  A two-phase evaluation of the 40-item Fatigue Impact scale (FIS): a qualitative evaluation of content and face validity using expert opinion (n=30) and a modified Delphi technique; a quantitative psychometric evaluation of internal and external construct validity of data from 333 people with multiple sclerosis using traditional and modern methods.
  
  
  RESULTS
  Qualitative evaluation did not support content or face validity of the FIS. Expert opinion agreed with the subscale placement of 23 items (58%), and classified all 40 items as being non-specific to fatigue impact. Nevertheless, standard quantitative psychometric evaluations implied, largely, FIS subscales were reliable and valid.
  
  
  CONCLUSIONS
  Standard quantitative 'psychometric' evaluations of PRO instrument validity can be misleading. Evaluation of existing PRO instruments requires both qualitative and statistical methods. Development of new PRO instruments requires stronger conceptual underpinning, clearer definitions of the substantive variables for measurement and hypothesis-testing experimental designs.
authors.affiliation: The Department of Clinical Neuroscience, Plymouth University Peninsula Schools of Medicine and Dentistry, Devon, UK.
authors.forename: Jeremy
authors.initials: J
authors.lastname: Hobart
authors.forename: Stefan
authors.initials: S
authors.lastname: Cano
authors.forename: Rachel
authors.initials: R
authors.lastname: Baron
authors.forename: Alan
authors.initials: A
authors.lastname: Thompson
authors.forename: Steven
authors.initials: S
authors.lastname: Schwid
authors.forename: John
authors.initials: J
authors.lastname: Zajicek
authors.forename: David
authors.initials: D
authors.lastname: Andrich
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003071:Cognition; D004185:Disability Evaluation; D005221:Fatigue / Q000209:etiology* / Q000503:physiopathology / Q000523:psychology / Q000534:rehabilitation*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000150:complications* / Q000503:physiopathology / Q000523:psychology / Q000534:rehabilitation*; D009460:Neurologic Examination; D063868:Patient Outcome Assessment; D017060:Patient Satisfaction; D010807:Physical Endurance; D011594:Psychometrics; D015203:Reproducibility of Results; D057566:Self Report; D012919:Social Behavior; D016896:Treatment Outcome; D055815:Young Adult
keywords: Patient-reported outcome measurement instruments; Rasch measurement theory; fatigue; multiple sclerosis; psychometric methods; rating scales