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

Monetary and other incentives may increase recruitment to randomised controlled trials. 2x2 factorial 'study within a trial' of including a pen and/or £5 (GBP) in cash with a postal recruitment pack to increase the number of participants randomised into the host trial ('Gentle Years Yoga') for older adults with multimorbidity. Secondary outcomes: return, and time to return, of screening form, and the cost per additional participant randomised. Binary data were analysed using logistic regression and time to return using Cox proportional hazards regression.  818 potential host trial participants were included. Between those sent a pen (n=409) and not sent a pen (n=409), there was no evidence of a difference in the proportion of participants randomised (15 (3.7%) 11 (2.7%); OR 1.38, 95% CI 0.63-3.04), in returning a screening form (66 (16.1%) 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). Between those sent £5 (n=409) and not sent £5 (n=409), there was no evidence of increased randomisation (14 (3.4%) 12 (2.9%); OR 1.18, 95% CI 0.54-2.57), but more screening forms were returned (77 (18.8%) 50 (12.2%); OR 1.67, 95% CI 1.13-2.45) and there was decreased time to return screening form (HR 1.56, 95% CI 1.09-2.22). No significant interaction between the interventions was observed. The cost per additional participant randomised was £32 and £1000 for the pen and £5, respectively. A small, monetary incentive did not result in more participants being randomised into the host trial but did encourage increased and faster response to the recruitment invitation. Since it is relatively costly, we do not recommend this intervention for use to increase recruitment in this population. Pens were cheaper but did not provide evidence of benefit.