A simulation study provided sample size guidance for differential item functioning (DIF) studies using short scales

Neil W. Scott, Peter M. Fayers, Neil K. Aaronson, Andrew Bottomley, Alexander de Graeff, Mogens Groenvold, Chad Gundy, Michael Koller, Morten A. Petersen, Mirjam A. G. Sprangers, EORTC Quality of Life Group; Quality of Life Cross-Cultural Meta-Analysis Group

Research output: Contribution to journalArticlepeer-review

109 Citations (Scopus)


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.

Study Design and setting: 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.

Conclusion: 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. (C) 2008 Elsevier Inc. All rights reserved.

Original languageEnglish
Pages (from-to)288-295
Number of pages8
JournalJournal of Clinical Epidemiology
Issue number3
Publication statusPublished - Mar 2009


  • Health-related quality of life
  • Differential item functioning
  • Ordinal logistic regression
  • Short scales
  • Computer simulations
  • Floor effects
  • mantel-haenszel procedures
  • logistic-regression
  • health applications
  • questionnaire
  • equivalence
  • invariance
  • QLQ-C30
  • bias


Dive into the research topics of 'A simulation study provided sample size guidance for differential item functioning (DIF) studies using short scales'. Together they form a unique fingerprint.

Cite this