title: Confirmatory factor analysis of the thyroid-related quality of life questionnaire ThyPRO.
abstract: OBJECTIVE
  Thyroid diseases are prevalent and chronic. With treatment, quality of life is restored in most, but not all patients. Construct validity of the thyroid-related quality of life questionnaire, ThyPRO, has been established by multi-trait scaling, but not evaluated with more elaborate methods. The purpose of the present study was to evaluate dimensionality of the ThyPRO scales and to attempt to understand possible item misfit through structural equation modeling for categorical data.
  
  
  METHODS
  The current 85-item version of ThyPRO consists of 13 scales, covering domains of physical (4 scales) and mental (2 scales) symptoms, function and well-being (3 scales) and participation/social function (4 scales). The data were collected from a cross-sectional sample of 907 thyroid patients. One-factor confirmatory models were fitted to each scale, and evaluated by model fit statistics (comparative fit index >0.95, root mean square error of approximation <0.08), magnitude of factor loadings, model residual correlations and modification indices (MI). Indications of multi-dimensionality were tested in bi-factor models. Possible item misfit was evaluated in a combined, investigational model.
  
  
  RESULTS
  Each ThyPRO scale was adequately represented by a unidimensional model after minor revisions. Eleven items were identified in the unidimensional models as potentially misfitting and were investigated further by multidimensional modeling.
  
  
  CONCLUSIONS
  Elaborate psychometric modeling supported the construct validity of the ThyPRO. However, 11 potentially misfitting items and 18 items with local dependence to other items are candidates for removal in future item reduction processes.
authors.forename: Torquil
authors.initials: T
authors.lastname: Watt
authors.forename: Mogens
authors.initials: M
authors.lastname: Groenvold
authors.forename: Nina
authors.initials: N
authors.lastname: Deng
authors.forename: Barbara
authors.initials: B
authors.lastname: Gandek
authors.forename: Ulla
authors.initials: U
authors.lastname: Feldt-Rasmussen
authors.forename: Åse Krogh
authors.initials: ÅK
authors.lastname: Rasmussen
authors.forename: Laszlo
authors.initials: L
authors.lastname: Hegedüs
authors.forename: Steen Joop
authors.initials: SJ
authors.lastname: Bonnema
authors.forename: Jakob Bue
authors.initials: JB
authors.lastname: Bjorner
mesh_terms: D000203:Activities of Daily Living; D000328:Adult; D000368:Aged; D003430:Cross-Sectional Studies; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D063868:Patient Outcome Assessment; D010549:Personal Satisfaction; D011594:Psychometrics; D011788:Quality of Life; D058992:Social Participation; D011795:Surveys and Questionnaires; D013959:Thyroid Diseases