6. Item Response Theory and Differential Item Functioning

  1. Peter M. Fayers1,2 and
  2. David Machin3,4

Published Online: 8 MAR 2002

DOI: 10.1002/0470846283.ch6

Quality of Life: Assessment, Analysis and Interpretation

Quality of Life: Assessment, Analysis and Interpretation

How to Cite

Fayers, P. M. and Machin, D. (2000) Item Response Theory and Differential Item Functioning, in Quality of Life: Assessment, Analysis and Interpretation, John Wiley & Sons, Ltd, Chichester, UK. doi: 10.1002/0470846283.ch6

Author Information

  1. 1

    Medical Research Council Clinical Trials Unit, London, UK

  2. 2

    Unit of Applied Clinical Research, Norwegian University of Science and Technology, Trondheim, Norway

  3. 3

    NMRC Clinical Trials & Epidemiology Research Unit, Singapore

  4. 4

    School of Health and Related Research, University of Sheffield, UK

Publication History

  1. Published Online: 8 MAR 2002
  2. Published Print: 18 APR 2000

ISBN Information

Print ISBN: 9780471968610

Online ISBN: 9780470846285

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Keywords:

  • item difficulty;
  • item discrimination;
  • logistic regression;
  • logistic IRT model;
  • Guttman scales;
  • adaptive testing

Summary

In contrast to the traditional psychometrics, item response theory introduces a different underlying model for the responses to questions. It is now assumed that patients with a particular level of QoL or functioning will have a certain probability of responding positively to each question. This probability will depend upon the “difficulty” of the item in question. For example, many patients with cancer might respond “yes” to “easy” questions such as “Do you have pain?” but only patients with a high level of pain are likely to reply “yes” to the more “difficult” question “Have you got very much pain?” This chapter explains the role of item response models, and how to fit them. Use of these models to examine the psychometric properties of QoL scales, and in particular differential item functioning, is also described.