Pain
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The generalisability of randomised controlled trials will be compromised if markers of treatment outcome also affect trial recruitment. In a large trial of chronic widespread pain, we aimed to determine the extent to which randomised participants represented eligible patients, and whether factors predicting randomisation also influenced trial outcome. Adults from 8 UK general practices were surveyed to determine eligibility for a trial of 2 interventions (exercise and cognitive behavioural therapy [CBT]). ⋯ Adjusting for this selection bias decreased the treatment effect associated with exercise and CBT but increased that observed for combined therapy. All were associated with changes in numbers needed to treat. This has important implications for the design and interpretation of pain trials generally.