title: Using Rasch analysis to revisit the validity of the Cataract TyPE Spec instrument for measuring cataract surgery outcomes.
abstract: OBJECTIVE
  To assess the psychometric properties of the Cataract TyPE Specification (Spec) questionnaire using the Rasch model.
  
  
  METHODS
  Flinders Medical Centre, Adelaide, Australia.
  
  
  METHODS
  The 12-item Cataract TyPE Spec questionnaire was self-administered to patients drawn from the cataract surgery waiting list. The questionnaire and its 5 subscales were assessed for fit to the Rasch model. Response category performance, item-fit targeting, unidimensionality (using principal components analysis), and differential item functioning were assessed. A shortened version (11 items) was tested for criterion validity by determining correlation with a global rating of vision question.
  
  
  RESULTS
  Two hundred ninety-four patients responded to the questionnaire. The response categories for each question functioned as intended. Person-separation reliability was high (0.90). Deletion of 1 misfitting item (nighttime driving) improved overall model fit. The principal components analysis of the residuals demonstrated unidimensionality for the 11-item Cataract TyPE Spec and 2 subscales. However, items were targeted to a less able population. Only 2 subscales (near vision and glare) were valid. There was a good statistically significant correlation between the Likert-scored global rating of vision and the Rasch-scaled Cataract TyPE Spec score (r = -0.66, P<.0001), suggesting good criterion validity.
  
  
  CONCLUSIONS
  With minor modifications, the Cataract TyPE Spec questionnaire and its glare and near-vision subscales were good measures of visual functioning in the cataract patient. Additional items to suit the more able, including patients having second-eye surgery, could improve the measurement properties.
authors.affiliation: NH&MRC 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: D002386:Cataract / Q000503:physiopathology; D002387:Cataract Extraction; D004185:Disability Evaluation; D006801:Humans; D019591:Pseudophakia / Q000503:physiopathology; D011594:Psychometrics / Q000379:methods*; D011788:Quality of Life; D015203:Reproducibility of Results; D016896:Treatment Outcome