A 2x2 randomised factorial SWAT of the use of a pen and small, financial incentive to improve recruitment rates in a randomised controlled trial of yoga for older adults with multimorbidity.

Background:  Monetary and other incentives may increase recruitment to randomised controlled trials. Methods: This was a 2x2 factorial 'study within a trial' of including a pen and/or £5 with a postal recruitment pack to improve randomisation rate (primary outcome) into the host Gentle Years Yoga trial in older adults with multimorbidity. Secondary outcomes: return, and time to return, of screening form, and the cost per additional participant recruited. Binary data were analysed using logistic regression and time to return data using Cox proportional hazards regression.  Results:  818 potential host trial participants included. Between those sent a pen (n=409) and not sent a pen (n=409), there was no evidence of a difference in the likelihood of being randomised (15 (3.7%) versus 11 (2.7%); OR 1.38, 95% CI 0.63-3.04), in returning a screening form (66 (16.1%) versus 61 (14.9%); OR 1.10, 95% CI 0.75-1.61) nor in time to return the screening form (HR 1.09, 95% CI 0.77-1.55). There was evidence of improved screening return rates (77 (18.8%) versus 50 (12.2%); OR 1.67, 95% CI 1.13-2.45) and time to return screening form (HR 1.56, 95% CI 1.09-2.22) but not randomisation (14 (3.4%) versus 12 (2.9%); OR 1.18, 95% CI 0.54-2.57) in those sent £5 (n=409) compared with those not sent £5 (n=409). No significant interaction effects between the interventions were observed. The cost per additional participant recruited was £32 for the pen and £1000 for the £5 incentive. Conclusion:  Including a small, monetary incentive encouraged increased and faster response to the recruitment invitation but did not result in more participants being randomised into the host trial. Since it is relatively costly, we do not recommend this intervention for use to increase recruitment in this population. Pens are cheaper but did not provide evidence of benefit. Further studies may be required.