title: Rasch analysis reveals problems with multiplicative scoring in the macular disease quality of life questionnaire.
abstract: OBJECTIVE
  To evaluate validity and psychometric characteristics of the Macular Disease Quality of Life questionnaire (MacDQoL), a multiplicative rating scale designed to measure vision-related quality of life (VRQoL) in macular diseases and age-related macular degeneration (AMD).
  
  
  METHODS
  Cross-sectional study.
  
  
  METHODS
  We included 108 patients with neovascular AMD at baseline before ranibizumab treatment.
  
  
  METHODS
  The psychometric properties of the MacDQoL were assessed using Rasch analysis, exploring key indices such as response category functioning, instrument unidimensionality, discriminant ability, and targeting of item difficulty to patient ability.
  
  
  METHODS
  Measurement characteristics of the MacDQoL.
  
  
  RESULTS
  In the MacDQoL's native form, the majority of response categories were underutilized and thresholds disordered. This could not be remedied without eliminating the importance ratings owing to the ambiguous nature of the response categories. Scaling problems were resolved by using the impairment rating scale only and collapsing response categories to 4. However, the MacDQoL was multidimensional, necessitating the omission of a number of items and splitting it into an activity limitation and mobility and a socioemotional well-being scale. This improved the psychometric parameters of the revised MacDQoL, although no correlation with clinical measures such as visual acuity was found.
  
  
  CONCLUSIONS
  The multiplicative rating scale of the MacDQoL is flawed and does not provide scientific measurement of VRQoL. Measurement can be restored with a series of revisions to the instrument. This study reinforces the importance of considering rating scale design when choosing patient reported outcomes instruments for healthcare research.
authors.affiliation: Center for Eye Research Australia, University of Melbourne, Royal Victorian Eye and Ear Hospital, Melbourne, Australia. robertfinger@gmx.net
authors.forename: Robert P
authors.initials: RP
authors.lastname: Finger
authors.forename: Eva
authors.initials: E
authors.lastname: Fenwick
authors.forename: Konrad
authors.initials: K
authors.lastname: Pesudovs
authors.forename: Manjula
authors.initials: M
authors.lastname: Marella
authors.forename: Ecosse L
authors.initials: EL
authors.lastname: Lamoureux
authors.forename: Frank G
authors.initials: FG
authors.lastname: Holz
mesh_terms: D000368:Aged; D020533:Angiogenesis Inhibitors / Q000627:therapeutic use; D061067:Antibodies, Monoclonal, Humanized / Q000627:therapeutic use; D003430:Cross-Sectional Studies; D005260:Female; D005451:Fluorescein Angiography; D006801:Humans; D007802:Language; D008297:Male; D011594:Psychometrics / Q000295:instrumentation*; D011788:Quality of Life; D000069579:Ranibizumab; D015203:Reproducibility of Results; D012164:Retinal Diseases / Q000188:drug therapy / Q000523:psychology*; D018434:Sickness Impact Profile; D011795:Surveys and Questionnaires; D014786:Vision Disorders / Q000523:psychology*; D014792:Visual Acuity / Q000502:physiology; D057135:Wet Macular Degeneration / Q000188:drug therapy / Q000523:psychology
chemical_list: D020533:Angiogenesis Inhibitors; D061067:Antibodies, Monoclonal, Humanized; D000069579:Ranibizumab