Mike: Hello, I'm Mike Clarke, podcast editor for the Cochrane Library. Most Cochrane Reviews take their evidence from randomized trials of the effects of healthcare interventions, and these trials need to be efficient and reliable. A challenge for many trials is ensuring that people who agree to join the study are followed up, so that their outcomes are measured. An updated Cochrane Methodology Review from March 2021 looks at research into ways to improve this retention, and our Podcast Editor, Mike Clarke, spoke with lead author, Katie Gillies, from the University of Aberdeen in Scotland about the review in this podcast.

Hello Katie, thanks very much both for doing this important update and for taking part in this podcast. Let's begin with a few words about the need to do the review?

Katie: Thanks Mike. It's good to be here. Our starting point was that there is very little evidence of what might encourage people to stay in a trial till the end and we wanted to bring as much of the relevant research on this retention together. This might include strategies directed at the participants, such as providing a gift voucher for attending a clinic or returning a questionnaire, or strategies directed at staff involved in trial retention, such as different ways of collecting data.
In terms of the scale of the problem, well, all trials need to recruit and retain patients, so retention is a concern for almost every single trial. Right now, there are tens of thousands of open trials worldwide, needing many millions of people to participate and complete the trial if they are to produce reliable results. So, identifying effective strategies to keep people in these and future trials till the end is really important.

Mike: With that in mind, did you find what you wanted to find?

Katie: Yes and no! We found over 70 studies that evaluated some type of strategy to encourage participants to complete a trial. So, on the one hand that's great because it means there is quite a bit of activity in this area of research. But, on the other hand, many of the strategies are so different that we could not combine them in summary analyses. It would be like comparing apples and chairs, which means that we can't conclude much about the results from existing studies but what we can say with certainty is where future efforts should focus given that there is some encouraging evidence.

Mike: Looking at the evidence you did find, what stood out in this update?

Katie: One thing was that the earlier version of this review concluded that monetary incentives were effective at improving the response to postal questionnaires but using GRADE to assess the evidence in our update has highlighted that we can't be certain about this due to some issues with the quality of the included studies.
That example also illustrates that there has been a lot of focus on studies to improve response to questionnaires and much less on other important areas for retention. For example, out of the 70 included studies, there are only two studies targeting attendance at clinic visits and only two that targeted trial staff rather than participants. This needs to be addressed.
Another area that proved quite frustrating to deal with was that many studies evaluated multiple interventions within the same evaluation. This makes it difficult to try and untangle which intervention is having any effect on outcomes and shows that better design and reporting of these studies is needed.

Mike: So, what would be your key messages for research teams studying retention?

Katie: Firstly, they need to work collaboratively to conduct high quality studies, with multiple replications across multiple trials at the same time, and report these evaluations clearly. In terms of where effort should be focused, we suggest four key areas:
First, testing monetary incentives to show if, for example, sending a £5 gift voucher along with a postal questionnaire will improve response.
The second area is return postage. Several return postage strategies have been tested for postal questionnaire and ensuring that participants are not out of pocket is key. However, we'd like to see trialists make more use of their patient partners in choosing the strategies to test and use.
Next, pens. Several studies have evaluated including a pen with a postal questionnaire to improve response and this looks promising but further studies, especially in trials in younger populations and men are required.
Finally, electronic reminders. We've found that emails or text messages reminding people that a questionnaire is on its way may improve postal questionnaire return but we need high quality replications to be sure about this.
In planning future research, it's also worth remembering that many of the strategies are low to minimal cost to and easy to implement when they are being evaluated. Also, here in the UK, our main trial funder, the National Institute for Health Research, encourages researchers to embed a methodology evaluation into the trials they fund, and testing a retention strategy would be a great way to do this.

Mike: Thanks very much Katie. That all sounds great with important opportunities for improving this evidence base in the future. Do you have any final words of advice, and how can listeners get hold of your full review?

Katie: Thanks Mike. I'd like to say to all the researchers who are listening: there's no excuse not to evaluate a strategy in your next trial – it's probably more cost effective than doing what you would normally and not evaluating it! And, to find the review, people should go online to Cochrane Library dot com and search for ‘retention in trials'.