title: A simulation study provided sample size guidance for differential item functioning (DIF) studies using short scales.
abstract: OBJECTIVE
  Differential item functioning (DIF) analyses are increasingly used to evaluate health-related quality of life (HRQoL) instruments, which often include relatively short subscales. Computer simulations were used to explore how various factors including scale length affect analysis of DIF by ordinal logistic regression.
  
  
  METHODS
  Simulated data, representative of HRQoL scales with four-category items, were generated. The power and type I error rates of the DIF method were then investigated when, respectively, DIF was deliberately introduced and when no DIF was added. The sample size, scale length, floor effects (FEs) and significance level were varied.
  
  
  RESULTS
  When there was no DIF, type I error rates were close to 5%. Detecting moderate uniform DIF in a two-item scale required a sample size of 300 per group for adequate (>80%) power. For longer scales, a sample size of 200 was adequate. Considerably larger sample sizes were required to detect nonuniform DIF, when there were extreme FEs or when a reduced type I error rate was required.
  
  
  CONCLUSIONS
  The impact of the number of items in the scale was relatively small. Ordinal logistic regression successfully detects DIF for HRQoL instruments with short scales. Sample size guidelines are provided.
authors.affiliation: Department of Public Health, University of Aberdeen, Aberdeen, 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: D003198:Computer Simulation; D005260:Female; D006801:Humans; D016015:Logistic Models; D008297:Male; D015233:Models, Statistical; D017063:Outcome Assessment, Health Care / Q000379:methods*; D011594:Psychometrics / Q000379:methods; D011788:Quality of Life / Q000523:psychology*; D018401:Sample Size; D014894:Weights and Measures