title: Testing item response theory invariance of the standardized Quality-of-life Disease Impact Scale (QDIS(®)) in acute coronary syndrome patients: differential functioning of items and test.
abstract: OBJECTIVE
  The Quality-of-life (QOL) Disease Impact Scale (QDIS(®)) standardizes the content and scoring of QOL impact attributed to different diseases using item response theory (IRT). This study examined the IRT invariance of the QDIS-standardized IRT parameters in an independent sample.
  
  
  METHODS
  The differential functioning of items and test (DFIT) of a static short-form (QDIS-7) was examined across two independent sources: patients hospitalized for acute coronary syndrome (ACS) in the TRACE-CORE study (N = 1,544) and chronically ill US adults in the QDIS standardization sample. "ACS-specific" IRT item parameters were calibrated and linearly transformed to compare to "standardized" IRT item parameters. Differences in IRT model-expected item, scale and theta scores were examined. The DFIT results were also compared in a standard logistic regression differential item functioning analysis.
  
  
  RESULTS
  Item parameters estimated in the ACS sample showed lower discrimination parameters than the standardized discrimination parameters, but only small differences were found for thresholds parameters. In DFIT, results on the non-compensatory differential item functioning index (range 0.005-0.074) were all below the threshold of 0.096. Item differences were further canceled out at the scale level. IRT-based theta scores for ACS patients using standardized and ACS-specific item parameters were highly correlated (r = 0.995, root-mean-square difference = 0.09). Using standardized item parameters, ACS patients scored one-half standard deviation higher (indicating greater QOL impact) compared to chronically ill adults in the standardization sample.
  
  
  CONCLUSIONS
  The study showed sufficient IRT invariance to warrant the use of standardized IRT scoring of QDIS-7 for studies comparing the QOL impact attributed to acute coronary disease and other chronic conditions.
authors.affiliation: Department of Quantitative Health Sciences, University of Massachusetts Medical School, Worcester, MA, 01655, USA, nndeng@gmail.com.
authors.forename: Nina
authors.initials: N
authors.lastname: Deng
authors.forename: Milena D
authors.initials: MD
authors.lastname: Anatchkova
authors.forename: Molly E
authors.initials: ME
authors.lastname: Waring
authors.forename: Kyung T
authors.initials: KT
authors.lastname: Han
authors.forename: John E
authors.initials: JE
authors.lastname: Ware
mesh_terms: D054058:Acute Coronary Syndrome / Q000523:psychology*; D000328:Adult; D000368:Aged; D002138:Calibration; D002908:Chronic Disease / Q000523:psychology; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011594:Psychometrics; D011788:Quality of Life; D011795:Surveys and Questionnaires