Abstract
BackgroundThe 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.MethodsThe 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.ResultsAfter 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.ConclusionsOverall, 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.
Highlights
The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) is a widely used patient reported outcome in osteoarthritis
Significant mean differences on WOMAC physical function were found with worse physical functioning for those in the lower social class group, the more deprived Townsend group, those not working, in low mood group and being obese, similar differences to those found in other OA samples
The impact of the differential item functioning (DIF) items on BMI for physical functioning subscale and for age for the hip pain subscale appear less likely to change conclusions as use of the DIF free items did significantly reduce the level of significance, the actual differences were still highly significant. It appears that for the physical functioning subscale it would be advisable to analyse data taking into account DIF items when weight, gender or especially age effects, are the focus of interest
Summary
The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) is a widely used patient reported outcome in osteoarthritis. If respondents have the same underlying level of an attribute, does the measure give the same score in different populations or is it biased in some groups? It has been shown that for the Centre of Epidemiology Scale of Depression (CES-D), women are more likely than men to endorse an item about having crying spells even though they have the same underlying level of depression [4] This item exhibits bias with respect to gender and scores for women might be inflated compared to men. Item level analyses are seen as central to establishing measurement equivalence across subgroups of a population [6]
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