BackgroundRandomised trials (also referred to as randomised controlled trials or trials) are the optimal way to minimise bias in evaluating the effects of competing treatments, therapies and innovations in health care. It is important to achieve the required sample size for a trial, otherwise trialists may not be able to draw conclusive results leading to research waste and raising ethical questions about trial participation. The reasons why potential participants may accept or decline participation are multifaceted. Yet, the evidence of effectiveness of interventions to improve recruitment to trials is not substantial and fails to recognise these individual decision-making processes. It is important to synthesise the experiences and perceptions of those invited to participate in randomised trials to better inform recruitment strategies.ObjectivesTo explore potential trial participants views and experiences of the recruitment process for participation. The specific objectives are to describe potential participants perceptions and experiences of accepting or declining to participate in trials, to explore barriers and facilitators to trial participation, and to explore to what extent barriers and facilitators identified are addressed by strategies to improve recruitment evaluated in previous reviews of the effects of interventions including a Cochrane Methodology Review.Search methodsWe searched the Cochrane Library, Medline, Embase, CINAHL, Epistemonikos, LILACS, PsycINFO, ORRCA, and grey literature sources. We ran the most recent set of searches for which the results were incorporated into the review in July 2017.Selection criteriaWe included qualitative and mixed-methods studies (with an identifiable qualitative component) that explored potential trial participants experiences and perceptions of being invited to participate in a trial. We excluded studies that focused only on recruiters perspectives, and trials solely involving children under 18 years, or adults who were assessed as having impaired mental capacity.Data collection and analysisFive review authors independently assessed the titles, abstracts and full texts identified by the search. We used the CART (completeness, accuracy, relevance, timeliness) criteria to exclude studies that had limited focus on the phenomenon of interest. We used QSR NVivo to extract and manage the data. We assessed methodological limitations using the Critical Skills Appraisal Programme (CASP) tool. We used thematic synthesis to analyse and synthesise the evidence. This provided analytical themes and a conceptual model. We used the GRADE-CERQual (Confidence in the Evidence from Reviews of Qualitative research) approach to assess our confidence in each finding. Our findings were integrated with two previous intervention effectiveness reviews by juxtaposing the quantitative and qualitative findings in a matrix.Main resultsWe included 29 studies (published in 30 papers) in our synthesis. Twenty-two key findings were produced under three broad themes (with six subthemes) to capture the experience of being invited to participate in a trial and making the decision whether to participate. Most of these findings had moderate to high confidence. We identified factors from the trial itself that influenced participation. These included how trial information was communicated, and elements of the trial such as the time commitment that might be considered burdensome. The second theme related to personal factors such as how other people can influence the individuals decision; and how a personal understanding of potential harms and benefits could impact on the decision. Finally, the potential benefits of participation were found to be key to the decision to participate, namely personal benefits such as access to new treatments, but also the chance to make a difference and help others. The conceptual model we developed presents the decision-making process as a gauge and the factors that influence whether the person will, or will not, take part.Authors' conclusionsThis qualitative evidence synthesis has provided comprehensive insight into the complexity of factors that influence a person's decision whether to participate in a trial. We developed key questions that trialists can ask when developing their recruitment strategy. In addition, our conceptual model emphasises the need for participant-centred approaches to recruitment. We demonstrated moderate to high level confidence in our findings, which in some way can be attributed to the large volume of highly relevant studies in this field. We recommend that these insights be used to direct or influence or underpin future recruitment strategies that are developed in a participant-driven way that ultimately improves trial conduct and reduces research waste.Plain language summaryWhat factors influence a person's decision whether or not to take part in a randomised trial?What is the aim of this review?Randomised trials are needed for understanding if and how different healthcare interventions (such as medicines, types of surgeries, health promotion activities, etc.) work or not. Getting people to take part in trials can be difficult and if not enough people participate, then the trial will not provide the information that it set out to. By learning more about what influences a persons decision to take part in a trial, we can provide advice on how best to include people in trials.To answer this question, we brought together the findings of 29 studies reporting the views and experiences of people who had been invited to take part in a randomised trial.Key messagesSeveral factors influence a persons decision to take part in a trial including: how the trial is set up and communicated; peoples own personal circumstances; and the potential benefits of participation. It is important that those working in trials take these into account when inviting people to participate. It is important that this is done in a manner that recognises that all people are different and may consider the trial in a different way.What was studied in the review?We searched for studies that examined the views of those who had been invited to take part in a randomised trial. We included studies with people who had agreed to take part as well as those who had decided not to take part. We included studies published since the start of the year 2000.Our searches identified 29 studies (published in 30 papers) to include in this review. Sixteen studies were conducted in the UK, six in other European countries, three in the USA, one each in Canada, Australia, New Zealand and Tanzania. The trials that people had been invited to take part in were for cancer (n = 7); pregnancy and childbirth (n = 5); medicine and surgery (n = 11); mental health (n = 2); and health promotion (n = 4).Our review pointed to three main factors that influenced whether potential participants agreed to take part in a trial or not. We judged the following findings to have moderate to high confidence.What are the main findings?People preferred to be invited in a face-to-face setting, with information communicated clearly. Written information was also useful. The timing of the invitation is important because potential participants could find it difficult to recognise the care they usually receive and care that would be provided as part of the trial.Commitment to the trial can make people worried about participating. Some believed that extra appointments and the time involved would be a burden. Sometimes people are offered money as a way to recompense them for their commitment. Payment was welcomed by some, but was not seen as a very important factor that influences their decision.If someone feels healthy, they may not wish to risk their health by taking part in a trial. However, if someone feels unwell, they may not want to risk making their health worse. On the other hand, someone who is healthy or very ill may feel they have nothing to lose by taking part in a trial, so it is not just about how healthy someone is but rather how they feel about their own health.Also, the persons doctor or nurse may say something that influences their decision, as can something said by family, friends or in the media. It is important for the people recruiting someone to know who has influence when that person is making their decisions.People are influenced by the chance of improvement, the chance to feel better if the therapy or treatment works, or the opportunity to make a difference by helping others in the future.Inviting people to take part in a trial should be done in a way that considers each person individually, because there is no one size fits all when it comes to making this decision.How up-to-date is this review?This review includes studies published up to 1 June 2017.