Abstract
Introduction The reduced WOMAC function scale has been developed and initial validity performed. However, further validation and recommendations for the treatment of missing values is required. The aim of this study is to further assess the validity of the reduced function scale of the WOMAC and recommend a protocol for the treatment of missing values.
Method Further validation of the reduced scale was performed via a cross-over study of 100 pre-operative total joint replacement patients, each being randomised to receive either the full or reduced scale along with the pain scale, and then the alternate version upon admission. Data utilised in the development of the reduced scale was used to develop a missing value protocol, where the number of valid responses for several protocols was examined, as well as comparison of the means and standard deviations. Of the consenting 100 patients, 66 continued onto admission. The median time between administrations of the questionnaires was 14 days (range zero to 72 days).
Results There was no significant difference between pain scores for each questionnaire using the paired t-test (p=0.56). Similarly, there was no significant difference between the full and reduced function scales (p=0.65). The standard protocol for the full scale is that if there are four or more missing items, the patient’s response is invalid. But when there are one to three items missing, the average value for the sub-scale is substituted in lieu of these missing values. Examining the frequencies of valid responses, means and standard deviations when using different missing value protocols (none missing, zero or one, up to two and up to three missing), indicated that there was no substantial benefit between the ‘up to two’ missing and ‘up to three’ missing response protocols. However, for this small gain, the supposition that the completed items are representative of the missing ones rises from 29% (two of seven items) to 43% (three of seven items) should be considered unacceptable.
Conclusions The reduced WOMAC function scale has been further validated. It is proposed that where three or more responses are missing, the patients response is regarded as invalid. Where there are one or two items missing, the average value for the sub-scale is substituted in lieu of these missing values.
The abstracts were prepared by Mr Jerzy Sikorski. Correspondence should be addressed to him at the Australian Orthopaedic Association, Ground Floor, William Bland Centre, 229 Macquarie Street, Sydney NSW 2000, Australia.
None of the authors have received any payment or consideration from any source for the conduct of this study.
