Abstract: We identified 68 eligible trials (24 new to this update) with more than 74,000 participants. There were 63 studies involving interventions aimed directly at trial participants, while five evaluated interventions aimed at people recruiting participants. All studies were in health care. We found 72 comparisons, but just three are supported by high-certainty evidence according to GRADE. 1. Open trials rather than blinded, placebo trials. The absolute improvement was 10% (95% CI 7% to 13%). 2. Telephone reminders to people who do not respond to a postal invitation. The absolute improvement was 6% (95% CI 3% to 9%). This result applies to trials that have low underlying recruitment. We are less certain for trials that start out with moderately good recruitment (i.e. over 10%). 3. Using a particular, bespoke, user-testing approach to develop participant information leaflets. This method involved spending a lot of time working with the target population for recruitment to decide on the content, format and appearance of the participant information leaflet. This made little or no difference to recruitment: absolute improvement was 1% (95% CI −1% to 3%). We had moderate-certainty evidence for eight other comparisons; our confidence was reduced for most of these because the results came from a single study. Three of the methods were changes to trial management, three were changes to how potential participants received information, one was aimed at recruiters, and the last was a test of financial incentives. All of these comparisons would benefit from other researchers replicating the evaluation. There were no evaluations in paediatric trials. We had much less confidence in the other 61 comparisons because the studies had design flaws, were single studies, had very uncertain results or were hypothetical (mock) trials rather than real ones. The literature on interventions to improve recruitment to trials has plenty of variety but little depth. Only 3 of 72 comparisons are supported by high-certainty evidence according to GRADE: having an open trial and using telephone reminders to non-responders to postal interventions both increase recruitment; a specialised way of developing participant information leaflets had little or no effect. The methodology research community should improve the evidence base by replicating evaluations of existing strategies, rather than developing and testing new ones.

Plain-language summary: The 68 included studies covered a very wide range of disease areas, including antenatal care, cancer, home safety, hypertension, podiatry, smoking cessation and surgery. Primary, secondary and community care were included. The size of the studies ranged from 15 to 14,467 participants. Studies came from 12 countries; there was also one multinational study involving 19 countries. The USA and UK dominated with 25 and 22 studies, respectively. The next largest contribution came from Australia with eight studies. Our search updated our 2010 review and is current to February 2015. We also identified six studies published after 2015 outside the search. The review includes 24 mock trials where the researchers asked people about whether they would take part in an imaginary trial. We have not presented or discussed their results because it is hard to see how the findings relate to real trial decisions.