title: Reengineering the glaucoma quality of life-15 questionnaire with rasch analysis.
abstract: OBJECTIVE
  To investigate, using Rasch analysis, whether the 15-item Glaucoma Quality of Life-15 (GQL-15) forms a valid scale and to optimize its psychometric properties.
  
  
  METHODS
  One hundred eighteen glaucoma patients (mean age, 65.7 years) completed the German-version of the GQL-15. Rasch analysis was performed to assess category function (how respondents differentiated between the response options), measurement precision (discriminative ability), unidimensionality (whether items measure a single construct), targeting (whether items are of appropriate difficulty for the sample), and differential item functioning (whether comparable subgroups respond differently to an individual item). Where any of these attributes were outside acceptable ranges, steps were taken to improve the instrument.
  
  
  RESULTS
  The five-response categories of the GQL-15 were well differentiated by respondents, as demonstrated by ordered and well-spaced category thresholds. The GQL-15 had an excellent measurement precision but demonstrated poor targeting of item difficulty to person ability and multidimensionality, indicating that it was measuring more than one construct. Removal of six misfitting items created a nine-item unidimensional instrument with good measurement precision and no differential item functioning but poor targeting. A new name, the Glaucoma Activity Limitation (GAL-9) questionnaire, is proposed for the short version, which better reflects the construct under measurement.
  
  
  CONCLUSIONS
  The GAL-9 has superior psychometric properties over the GQL-15. Its only limitation is poor targeting of item difficulty to person ability, which is an inevitable attribute of a vision-related activity limitation instrument for glaucoma patients, most of whom have only peripheral visual field defects and little difficulty with daily activities.
authors.affiliation: National Health and Medical Research Council (NH&MRC) Centre for Clinical Eye Research, Discipline of Optometry and Vision Science, Flinders Medical Centre and Flinders University of South Australia, South Australia, Australia.
authors.forename: Jyoti
authors.initials: J
authors.lastname: Khadka
authors.forename: Konrad
authors.initials: K
authors.lastname: Pesudovs
authors.forename: Colm
authors.initials: C
authors.lastname: McAlinden
authors.forename: Michaela
authors.initials: M
authors.lastname: Vogel
authors.forename: Marcus
authors.initials: M
authors.lastname: Kernt
authors.forename: Christoph
authors.initials: C
authors.lastname: Hirneiss
mesh_terms: D000203:Activities of Daily Living; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D004185:Disability Evaluation; D005260:Female; D005858:Germany; D005901:Glaucoma / Q000523:psychology*; D006801:Humans; D007802:Language; D008297:Male; D008875:Middle Aged; D008962:Models, Theoretical; D011594:Psychometrics; D011788:Quality of Life / Q000523:psychology*; D015203:Reproducibility of Results; D018434:Sickness Impact Profile; D011795:Surveys and Questionnaires; D014792:Visual Acuity / Q000502:physiology; D055815:Young Adult