Rasch analysis of the Western Ontario McMaster (WOMAC) Osteoarthritis Index: results from community and arthroplasty samples.
OBJECTIVE
This study is based on secondary analysis of Western Ontario McMaster Osteoarthritis Index (WOMAC) data from a community sample over 55 years and total hip or knee arthroplasty samples presurgery and 1-year postoperative.


METHODS
The WOMAC data were evaluated by Rasch analysis. Data were considered to fit the Rasch mathematical model for the pain and physical dimensions of the WOMAC if unidimensionality was confirmed by principle component analysis of the subscale and the residuals from the Rasch analysis, infit and outfit statistics were in the range of 0.80 to 1.20; if there was no differential item functioning based on gender or hip vs. knee subjects; and, if there was stability of the item logits across the three data samples.


RESULTS
A three-item pain dimension (excluding night pain and pain on standing) and a 14-item physical dimension (excluding heavy domestic duties, getting in and out of the bath and getting on and off the toilet) fit the Rasch model based on these criteria.


CONCLUSIONS
In evaluating existing health status questionnaires using Rasch methodology, it is important to evaluate relevant patient samples and longitudinal data when the measure is intended to evaluate change in status. By these criteria, a modified WOMAC questionnaire fits the Rasch model and has interval-level scaling properties.
Davis
A M
AM
Department of Physical Therapy, Graduate Department of Rehabilitation Science and Health Policy, Management and Evaluation, Toronto Rehabilitation Institute and the University of Toronto, Toronto, ON, Canada. davis.aileen@torontorehab.on.ca
Badley
E M
EM
Beaton
D E
DE
Kopec
J
J
Wright
J G
JG
Young
N L
NL
Williams
J I
JI
D000368:Aged; D019643:Arthroplasty, Replacement; D019644:Arthroplasty, Replacement, Hip; D019645:Arthroplasty, Replacement, Knee; D003627:Data Interpretation, Statistical; D005260:Female; D006304:Health Status; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D010003:Osteoarthritis / Q000534:rehabilitation / Q000601:surgery*; D010146:Pain; D012680:Sensitivity and Specificity; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires; D016896:Treatment Outcome