Validation and interval scale transformation of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) in patients undergoing knee arthroplasty, using the Rasch model.

Objectives: Interval scale reduce measurement bias compared to ordinal scale. We aimed to evaluate the fit of Western Ontario and McMaster Universities Osteoarthritis Index WOMAC) to the Rasch model and derive the transformation table for interval scale measurement.

Methods: Data from osteoarthritis patients listed for knee arthroplasty (KA) pre-operatively, and at 6- and 12- months post-operative was used.