title: Item response theory detected differential item functioning between healthy and ill children in quality-of-life measures.
abstract: OBJECTIVE
  To demonstrate the value of item response theory (IRT) and differential item functioning (DIF) methods in examining a health-related quality-of-life measure in children and adolescents.
  
  
  METHODS
  This illustration uses data from 5,429 children using the four subscales of the PedsQL 4.0 Generic Core Scales. The IRT model-based likelihood ratio test was used to detect and evaluate DIF between healthy children and children with a chronic condition.
  
  
  RESULTS
  DIF was detected for a majority of items but canceled out at the total test score level due to opposing directions of DIF. Post hoc analysis indicated that this pattern of results may be due to multidimensionality. We discuss issues in detecting and handling DIF.
  
  
  CONCLUSIONS
  This article describes how to perform DIF analyses in validating a questionnaire to ensure that scores have equivalent meaning across subgroups. It offers insight into ways information gained through the analysis can be used to evaluate an existing scale.
authors.affiliation: Thurstone Psychometric Laboratory, University of North Carolina, Davie Hall, CB #3270, Chapel Hill, NC 27599-3270, USA. langer@email.unc.edu
authors.forename: Michelle M
authors.initials: MM
authors.lastname: Langer
authors.forename: Cheryl D
authors.initials: CD
authors.lastname: Hill
authors.forename: David
authors.initials: D
authors.lastname: Thissen
authors.forename: Tasha M
authors.initials: TM
authors.lastname: Burwinkle
authors.forename: James W
authors.initials: JW
authors.lastname: Varni
authors.forename: Darren A
authors.initials: DA
authors.lastname: DeWalt
mesh_terms: D000293:Adolescent; D002648:Child; D002908:Chronic Disease / Q000523:psychology / Q000534:rehabilitation*; D006305:Health Status Indicators; D006801:Humans; D007398:Interpersonal Relations; D015233:Models, Statistical; D011788:Quality of Life; D011795:Surveys and Questionnaires