Exploring differential item functioning in the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)

Beth Pollard*, Marie Johnston, Diane Dixon

*Corresponding author for this work

Research output: Contribution to journalArticle

11 Citations (Scopus)
4 Downloads (Pure)

Abstract

Background: The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) is a widely used patient reported outcome in osteoarthritis. An important, but frequently overlooked, aspect of validating health outcome measures is to establish if items exhibit differential item functioning (DIF). That is, if respondents have the same underlying level of an attribute, does the item give the same score in different subgroups or is it biased towards one subgroup or another. The aim of the study was to explore DIF in the Likert format WOMAC for the first time in a UK osteoarthritis population with respect to demographic, social, clinical and psychological factors.

Methods: The sample comprised a community sample of 763 people with osteoarthritis who participated in the Somerset and Avon Survey of Health. The WOMAC was explored for DIF by gender, age, social deprivation, social class, employment status, distress, body mass index and clinical factors. Ordinal regression models were used to identify DIF items.

Results: After adjusting for age, two items were identified for the physical functioning subscale as having DIF with age identified as the DIF factor for 2 items, gender for 1 item and body mass index for 1 item. For the WOMAC pain subscale, for people with hip osteoarthritis one item was identified with age-related DIF. The impact of the DIF items rarely had a significant effect on the conclusions of group comparisons.

Conclusions: Overall, the WOMAC performed well with only a small number of DIF items identified. However, as DIF items were identified in for the WOMAC physical functioning subscale it would be advisable to analyse data taking into account the possible impact of the DIF items when weight, gender or especially age effects, are the focus of interest in UK-based osteoarthritis studies. Similarly for the WOMAC pain subscale in people with hip osteoarthritis it would be worthwhile to analyse data taking into account the possible impact of the DIF item when age comparisons are of primary interest.

Original languageEnglish
Article number265
Number of pages10
JournalBMC Musculoskeletal Disorders
Volume13
DOIs
Publication statusPublished - 29 Dec 2012

Fingerprint

Ontario
Osteoarthritis
Hip Osteoarthritis
Body Mass Index
Pain
Health Surveys
Social Class
Demography
Outcome Assessment (Health Care)
Psychology
Weights and Measures
Health
Population

Keywords

  • scale
  • osteoarthritis
  • quality-of-life
  • hip
  • psychometrics
  • rheumatoid-arthritis
  • measurement equivalence
  • validity
  • WOMAC
  • item bias
  • health-assessment questionnaire
  • outcome measures
  • differential item functioning
  • ordinal logistic-regression
  • clinical-trials
  • Rasch analysis

Cite this

Exploring differential item functioning in the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). / Pollard, Beth; Johnston, Marie; Dixon, Diane.

In: BMC Musculoskeletal Disorders, Vol. 13, 265, 29.12.2012.

Research output: Contribution to journalArticle

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abstract = "Background: The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) is a widely used patient reported outcome in osteoarthritis. An important, but frequently overlooked, aspect of validating health outcome measures is to establish if items exhibit differential item functioning (DIF). That is, if respondents have the same underlying level of an attribute, does the item give the same score in different subgroups or is it biased towards one subgroup or another. The aim of the study was to explore DIF in the Likert format WOMAC for the first time in a UK osteoarthritis population with respect to demographic, social, clinical and psychological factors.Methods: The sample comprised a community sample of 763 people with osteoarthritis who participated in the Somerset and Avon Survey of Health. The WOMAC was explored for DIF by gender, age, social deprivation, social class, employment status, distress, body mass index and clinical factors. Ordinal regression models were used to identify DIF items.Results: After adjusting for age, two items were identified for the physical functioning subscale as having DIF with age identified as the DIF factor for 2 items, gender for 1 item and body mass index for 1 item. For the WOMAC pain subscale, for people with hip osteoarthritis one item was identified with age-related DIF. The impact of the DIF items rarely had a significant effect on the conclusions of group comparisons.Conclusions: Overall, the WOMAC performed well with only a small number of DIF items identified. However, as DIF items were identified in for the WOMAC physical functioning subscale it would be advisable to analyse data taking into account the possible impact of the DIF items when weight, gender or especially age effects, are the focus of interest in UK-based osteoarthritis studies. Similarly for the WOMAC pain subscale in people with hip osteoarthritis it would be worthwhile to analyse data taking into account the possible impact of the DIF item when age comparisons are of primary interest.",
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