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: 2x2 factorial ‘study within a trial’ evaluating the effectiveness of including a pen and/or £5 with a postal recruitment pack to increase randomisation rates (primary outcome) into the host Gentle Years Yoga trial in older adults with multimorbidity.
Methods: Secondary outcomes were the return, and time to return, of a 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 were included. Between those sent a pen (n=409) and those not sent a pen (n=409), there was no evidence of a difference in the likelihood of being randomised (15 (3.7%) vs 11 (2.7%); OR 1.38, 95% CI 0.63 to 3.04), in returning a screening form (n=66 (16.1%) vs n=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%) vs 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%) vs 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.