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OBJECTIVES: (1) To develop trial protocols which promote the achievement of blind outcome assessment. (2) To report outcome assessor beliefs regarding group allocation at follow-up assessments. (3) To document and describe instances of unblinding occurring during the trial to assist and inform rehabilitation researchers and clinicians. DESIGN: Prospective longitudinal observational study. SETTING: An NHS Hospital Trust specializing in orthopaedic surgery. SUBJECTS: One hundred and seven patients participating in a prospective pragmatic randomized controlled trial investigating physiotherapy rehabilitation following total knee arthroplasty, plus three outcome assessors. INTERVENTIONS: A protocol was developed using available research and designed to minimize instances of unblinding during a physiotherapy rehabilitation trial. Administrative, office, patient and research staff procedures were included. MAIN MEASURES: Trial questionnaires measured blind outcome assessment responses at 3 and 12 months post surgery. The outcome assessor kept a field diary recording the events surrounding instances of unblinding. Data underwent descriptive and content analysis. RESULTS: Blind outcome assessment was believed successful for n = 74 (81.32%) assessments at 3-month follow-up, and n = 83 (91.21%) at 12 months. Forty instances (n = 28 participants) of unblinding were described in the field diary. While the main cause of unblinding was participants telling the outcome assessor, in 12.5% of events the assessor drew the wrong conclusion regarding group allocation. Not all unblinding events were remembered at subsequent assessments, even in this relatively small trial. CONCLUSIONS: Blind outcome assessment was considered achievable in this trial. Specific trial protocols enabled blinding beliefs to be reported and instances of unblinding to be described.

Original publication




Journal article


Clin rehabil

Publication Date





264 - 274


Arthroplasty, Replacement, Knee, Double-Blind Method, Female, Humans, Longitudinal Studies, Male, Osteoarthritis, Knee, Outcome and Process Assessment, Health Care, Physical Therapy Modalities, Prospective Studies, Randomized Controlled Trials as Topic, Reproducibility of Results, Single-Blind Method, Surveys and Questionnaires, United Kingdom