Increasing financial incentives can lower the cost of trial recruitment.

Monetary incentives are commonly used to help recruit trial participants. Some studies have found greater recruitment with larger incentives, while others have found smaller incentives more cost-effective in terms of cost per participant. As part of an implementation study, we compared the impact of four approaches to recruitment, three of which involved phone recruitment with varying financial incentives. Adding modest financial incentives reliably increased the recruitment ratio, and greater incentives increased recruitment more than smaller incentives. However, recruiters required less time to obtain agreement to participate when the greater incentive was offered, and these time savings made the greater incentive cost-saving relative to the smaller incentive and cost-effective relative to no incentive. Our results suggest the possibility of a "sweet spot" for financial incentives, and that trial designers should consider pilot-testing incentive levels in the context of their other recruitment costs to determine whether paying participants more may be cost-saving for trial sponsors.