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)). Amongst those eligible, logistic regression identified factors associated with reaching the randomisation step in the recruitment process. The main trial analysis was recomputed, weighting for the inverse of the likelihood of reaching the randomisation stage, and the numbers needed to treat were calculated for each treatment. Eight hundred eighty-four persons were identified as eligible for the trial, of whom 442 (50%) were randomised. Several factors were associated with the likelihood of reaching the randomisation stage: higher body mass index (odds ratio 1.99; 0.85 to 4.61); more severe/disabling pain (1.90; 1.21 to 2.97); having a treatment preference (2.11; 1.48 to 3.00); and expressing positivity about interventions offered (exercise 2.66; 1.95 to 3.62; CBT 3.20; 2.15 to 4.76). 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.

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