Research exploring monetary incentives for recruiting and retaining participants in randomised controlled trials: a...

Poor participant recruitment and retention in randomised controlled trials (RCTs) is a widespread challenge. Offering trial participants monetary incentives (e.g. cash or vouchers) may boost recruitment and retention; however, no reviews have systematically mapped the existing quantitative, qualitative, and mixed-method evidence on this topic. We therefore conducted a scoping review aiming to map the extent and type of available research examining monetary incentives for recruiting and retaining trial participants to identify critical areas where evidence remains sparse.

We included empirical studies examining the use of monetary incentives for recruiting or retaining participants in RCTs (excluding phase I trials). Case studies, protocols, and reviews without a systematic search strategy were excluded. Searches were run in the ORRCA database until the end of 2019 when it was last updated, and CINAHL, MEDLINE, Embase, and EconLit from 2020 onwards.

Out of 5421 identified records, 145 studies from 141 sources met our inclusion criteria. Monetary incentives were primarily studied in the context of trial retention (39%; 56/145) or trial participation more broadly (33%; 48/145), with many studies not specifying the format (61%; 88/145) or value (40%; 58/145) of monetary incentives in question. Additionally, the vast majority of studies were conducted in the USA (48%; 69/145) or UK (34%; 49/145), with very little research activity in low- and middle-income countries (LMICs) (9%; 13/145). Monetary incentives for recruiting or retaining trial participants were most often explored in the context of cross-sectional surveys (21%; 30/145), of which most involved members of the public being asked about hypothetical trial scenarios (53%; 16/30), rather than individuals actively invited to or involved in a trial.

This scoping review consolidates available research on monetary incentives for recruiting and retaining trial participants. Less research is needed involving members of the public being asked about monetary incentives for a hypothetical trial. Rather, we need more qualitative studies exploring the perspectives of trial staff responsible for their implementation, as well as randomised studies within a trial (SWATs) assessing cost-effectiveness alongside effectiveness. Furthermore, more research is needed exploring the use of monetary incentives in LMICs.