title: Differential item functioning (DIF) analyses of health-related quality of life instruments using logistic regression.
abstract: BACKGROUND
  Differential item functioning (DIF) methods can be used to determine whether different subgroups respond differently to particular items within a health-related quality of life (HRQoL) subscale, after allowing for overall subgroup differences in that scale. This article reviews issues that arise when testing for DIF in HRQoL instruments. We focus on logistic regression methods, which are often used because of their efficiency, simplicity and ease of application.
  
  
  METHODS
  A review of logistic regression DIF analyses in HRQoL was undertaken. Methodological articles from other fields and using other DIF methods were also included if considered relevant.
  
  
  RESULTS
  There are many competing approaches for the conduct of DIF analyses and many criteria for determining what constitutes significant DIF. DIF in short scales, as commonly found in HRQL instruments, may be more difficult to interpret. Qualitative methods may aid interpretation of such DIF analyses.
  
  
  CONCLUSIONS
  A number of methodological choices must be made when applying logistic regression for DIF analyses, and many of these affect the results. We provide recommendations based on reviewing the current evidence. Although the focus is on logistic regression, many of our results should be applicable to DIF analyses in general. There is a need for more empirical and theoretical work in this area.
authors.affiliation: Section of Population Health, University of Aberdeen, UK. n.w.scott@abdn.ac.uk
authors.forename: Neil W
authors.initials: NW
authors.lastname: Scott
authors.forename: Peter M
authors.initials: PM
authors.lastname: Fayers
authors.forename: Neil K
authors.initials: NK
authors.lastname: Aaronson
authors.forename: Andrew
authors.initials: A
authors.lastname: Bottomley
authors.forename: Alexander
authors.initials: A
authors.lastname: de Graeff
authors.forename: Mogens
authors.initials: M
authors.lastname: Groenvold
authors.forename: Chad
authors.initials: C
authors.lastname: Gundy
authors.forename: Michael
authors.initials: M
authors.lastname: Koller
authors.forename: Morten A
authors.initials: MA
authors.lastname: Petersen
authors.forename: Mirjam A G
authors.initials: MA
authors.lastname: Sprangers
mesh_terms: D003627:Data Interpretation, Statistical; D005260:Female; D006305:Health Status Indicators; D006801:Humans; D016015:Logistic Models; D008297:Male; D011594:Psychometrics; D011788:Quality of Life; D018401:Sample Size; D011795:Surveys and Questionnaires