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The Quebec Back Pain Disability Scale is a 20-item self-administered instrument designed to assess the level of functional disability in individuals with back pain. The scale was administered as part of a larger questionaire to a group of 242 back pain patients. Follow-up data were o btained after several days and after 2 to 6 months.The goal of this study was to determine whether the Quebec scale is a reliable, vaild, and responsive measure of disability in back pain, and to compare it with other disability scales.A number of functional disability scales for back pain are being used, but their conceptual validity is uncertain, unlike most published instruments,the Quebec scale was constructed using a conceptual approach to disability assessment and empirical methods of item development, analysis, and selection.The authors calculated test-retest and internal consistency coefficients, evaluated construct validity of the scale, and tested its responsiveness against a global index of change. Direct comparisons with the Roland, Oswestry, and SF-36 scales were carried out.Test-retest reliability was 0.92, and Cronbach's alpha coefficient was 0.96. The scale correlated as expected with other measures of disability, pain, medical history, and utilization variables, work-related variables, and socio-demographic characteristics. Significant changes in disability over time, and differences in change scores between patients that were expected to differ in the direction of change, were found.The Quebec scale can be recommended as an outcome measure in clinical trials, and for monitoring the progress of individual patients participating in treatment or rehabilitation programs.