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a Arthritis Research
Center and University of Kansas School of Medicine, Wichita, Kansas,
USA, b Merck and Co Inc, Whitehouse
Station, New Jersey, USA
Correspondence to: Dr F Wolfe, Arthritis Research Center, 1035 N Emporia, Suite 230, Wichita, KS 67214, USA.
Accepted for publication 26 April 1999
OBJECTIVE
Advances in
health measurement have led to the application of Rasch Item Response
Theory (IRT) analysis (Rasch analysis) to evaluate instruments
measuring health status and quality of life of patients, including the
Health Assessment Questionnaire and SF-36. This study investigated the
extent to which the Western Ontario MacMaster osteoarthritis
questionnaire (WOMAC) satisfies the Rasch model, particularly in
respect to unidimensionality, item separation, and linearity.
METHODSThe study
included a total of 2205 patients, 1013 with rheumatoid arthritis (RA),
655 with osteoarthritis of the knee or hip (OA), and 537 with
fibromyalgia. All patients completed the WOMAC as part of a
longitudinal study of rheumatic disease outcomes. To examine whether
the WOMAC pain and function scales each fits the Rasch model, the
Winsteps program was used to assess item difficulty, scale
unidimensionality, item separation, and linearity.
RESULTSAlthough the
WOMAC worked best in OA, regardless of disorder, both the pain and
function scales were unidimensional, had adequate item separation, and
had a long range (25-150) of linearity in the function scale. Several
functional items, however, had a high information weight fit (INFIT)
statistic, indicating poor fit to the model. These items included
"getting in and out of the bath" and "going down stairs."
CONCLUSIONThe WOMAC
generally satisfies the requirements of Rasch item response theory
across all disorders studied, and is an appropriate measure of lower
body function in OA, RA and fibromyalgia. Although some individual
items do not fit well, it is not likely that removing such items would
result in more than overall minimal differences, and it will be
difficult to remove traces of multidimensionality while keeping the
central constructs of progressive lower body musculoskeletal
abnormality intact. In addition, it is possible that a "purer",
still more unidimensional instrument would be less useful in clinical
trials and epidemiological studies by restricting the range of the scale.
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