The OKS-APQ was developed and tested in a comprehensive three stage process:
Stage 1 – Item generation
Semi-structured, in-depth interviews were carried out with 26 patients, with osteoarthritis (OA), attending orthopaedic or pre-admission clinics at a regional centre, either under consideration for knee replacement, or already awaiting Total Knee replacement (TKR) or Unicompartmental Knee Replacement (UKR) within the next four weeks, or had undergone TKR or UKR between two and 24 months previously.
A set of preliminary questionnaire items were then discussed with an expert panel and refined to produce a draft questionnaire. The draft questionnaire was then piloted with new patients in clinics in three different formats (all with five category Likert scale), a brief cognitive interview then followed and an independent formal assessment of readability and translatability was also conducted resulting in a draft 17-item questionnaire.
Stage 2 and 3 – Item reduction and scale generation; testing scale properties
A first postal study (n=73) was conducted using the 17 item questionnaire generated at stage one, along with the Oxford Knee score and the SF-36 generic health questionnaire (Mental Health Component Summary Score (MCS) and Physical Component Summary Score (PCS)). The resulting data was analysed to identify any issues with items and assess internal consistency (using Cronbach’s alpha). The formal process of item reduction used Exploratory Factor Analysis (EFA), Confirmatory factor Analysis (CFA) and Rasch analysis. Analyses were revisited to confirm the validity of the measure across methods.
Construct validity of the OKS-APQ was examined by correlation with the OKS, SF-36 and Knee and Functional American Knee Society Scores (AKSS) results. There were nine items removed in total: four items because of the high celling effect, another four in order to have clear factor structure (that excludes any item) and one item with more that 10% missing data.
The final eight-item questionnaire seems acceptable to patients, as demonstrated by high response rates (effect size 4.16). It has a high level of internal consistency and no redundant items. The OKS-APQ is designed to be used alongside the OKS and unlike the latter, is not recommended to be used as a stand-alone instrument.
Details of the development and validation of the OKS-APQ are described in:
Dawson J, Beard DJ, McKibbin H, Harris K, Jenkinson C, Price AJ. Development of a patient-reported outcome measure of activity and participation (the OKS-APQ) to supplement the Oxford knee score. J Bone Joint Surg Br. 2014 Mar;96-B(3):332-8
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