title: A Differential Item Functioning Analysis of the EQ-5D in Cancer.
abstract: OBJECTIVE
  To determine whether differential item functioning (DIF) was present in the EuroQol five-dimensional questionnaire (EQ-5D) used in cancer (non-small cell lung cancer and prostate cancer).
  
  
  METHODS
  The Partial Credit Model was applied to the three-level version of the EQ-5D with data obtained from four randomized controlled trials in prostate cancer and non-small cell lung cancer completed at baseline before treatment (N = 2213). DIF was assessed across cancer type (two levels), sex (two levels), and age group (three levels) using Mantel-Haenszel chi-square statistics and evaluated against the Educational Testing Service classification rules.
  
  
  RESULTS
  The presence of DIF was determined in 14 of 25 (56%) potential DIF contrasts in all the EQ-5D domains. Although mostly the DIF was categorized as either negligible (3 of 25 [12%]) or medium (7 of 25 [28%]), large DIF was observed in 4 of the 25 contrasts (16%). The mobility domain, in particular, showed consistently large DIF across cancer type, sex, and age.
  
  
  CONCLUSIONS
  Given the use of the instrument in health status assessments across conditions and interventions, these results may have significant implications for the EQ-5D in health economic evaluations. Further research is warranted to determine whether these results hold for other cancers.
authors.affiliation: York Health Economics Consortium, University of York, York, UK. Electronic address: adam.smith@york.ac.uk.
authors.forename: Adam B
authors.initials: AB
authors.lastname: Smith
authors.affiliation: York Trials Unit, Department of Health Sciences, University of York, York, UK.
authors.forename: Kim
authors.initials: K
authors.lastname: Cocks
authors.affiliation: AstraZeneca Pharmaceuticals Ltd., Macclesfield, Cheshire, UK.
authors.forename: David
authors.initials: D
authors.lastname: Parry
authors.affiliation: York Health Economics Consortium, University of York, York, UK.
authors.forename: Matthew
authors.initials: M
authors.lastname: Taylor
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D002289:Carcinoma, Non-Small-Cell Lung / Q000523:psychology*; D005260:Female; D006801:Humans; D008175:Lung Neoplasms / Q000523:psychology*; D008297:Male; D008875:Middle Aged; D011471:Prostatic Neoplasms / Q000523:psychology*; D011594:Psychometrics; D011788:Quality of Life; D016032:Randomized Controlled Trials as Topic; D012737:Sex Factors; D011795:Surveys and Questionnaires / Q000592:standards*; D055815:Young Adult
keywords: EQ-5D; Rasch; cancer; differential item functioning; oncology