title: The impact of cataract surgery questionnaire: re-evaluation using Rasch analysis.
abstract: OBJECTIVE
  The Impact of Cataract Surgery (ICS) questionnaire was designed to assess cataract surgery outcomes. The aim of this study was to describe the psychometric properties of the ICS questionnaire using the Rasch model in a cataract population.
  
  
  METHODS
  Ninety-one patients waiting to undergo cataract surgery in the first or second eye at the Flinders Medical Centre, Adelaide, South Australia self-administered the four-item ICS questionnaire. Rasch analysis was performed to assess behaviour of response categories, ability to differentiate between participants' visual abilities (person separation; minimum acceptable 2.0), if items measure a single underlying construct [i.e. unidimensionality assessed by fit statistics and further by principal components analysis (PCA)] and matching of item difficulty to participant ability (targeting; ideal < 0.5 logits). Adequate person separation was defined as basic requirement for a measure, failing which further assessment such as PCA was not performed.
  
  
  RESULTS
  The four-item ICS questionnaire did not meet the required measurement properties (person separation zero). Response categories did not behave as intended, requiring the collapsing of categories for one item (read ordinary newspaper-size print). One item misfit (estimating distance) indicating that it was not measuring the same construct as other items. However, person separation failed to improve following the deletion of this item. Targeting was -0.46 logits, indicating that the item difficulty was well suited to the visual abilities of the participants.
  
  
  CONCLUSIONS
  In its present form, the ICS is unsuitable for visual disability assessment in patients awaiting cataract surgery. Other, better visual function questionnaires are available and preferred.
authors.affiliation: National Health and Medical Research Council, Centre for Clinical Eye Research, Department of Ophthalmology, Flinders Medical Centre and Flinders University of South Australia, Bedford Park, South Australia, Australia.
authors.forename: Vijaya K
authors.initials: VK
authors.lastname: Gothwal
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Wright
authors.forename: Ecosse L
authors.initials: EL
authors.lastname: Lamoureux
authors.forename: Konrad
authors.initials: K
authors.lastname: Pesudovs
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002386:Cataract / Q000523:psychology*; D002387:Cataract Extraction / Q000523:psychology*; D004185:Disability Evaluation; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011594:Psychometrics / Q000379:methods / Q000592:standards*; D015203:Reproducibility of Results; D013021:South Australia; D011795:Surveys and Questionnaires / Q000592:standards*; D019987:Persons with Visual Disabilities / Q000523:psychology*