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Participant retention refers to keeping enrolled participants in a trial for the duration of the study. Retention is key for several reasons. Studies will be powered using sample size estimates, to ensure that a sufficient number of participants are enrolled for valid statistical inferences to be made about the results. Poor retention will reduce the power of a study, but selective drop-out will also bias the results of a study. If, for example, participants who experience good outcomes decide that they no longer need to take part, the remaining participants may have poorer outcomes and thus bias the results towards poor outcomes.


Conversely, participants who have poor outcomes in the trial may drop out biasing results towards good outcomes. This is especially problematic where the drop out is not random by study arm, but biased either towards or against the intervention arm of the study.


Poor retention rates, high drop out of participants, are likely to invalidate study assumptions and lead to trials that are unable to show effects statistically. Studies with low power or biased retention could be seen to be unethical, and so high participant retention can be seen to be a key ethical issue.


If participants are properly informed prior to giving voluntary consent to taking part in a study, there is no reason why they should want to drop out of a study early. That is, participants will understand that the risks and benefits of taking part, the importance of seeing the trial to its conclusion, and have ample time to decide whether they are able and willing to commit to the time and other personal sacrifices required to take part in a study. However, for whatever unanticipated reasons, there will always be some participants who wish to terminate their participation early, and they are free to do so as it would be unethical to keep participants in a trial if they are no longer willing to take part.


Good retention, thus, starts with a thorough and clear informed consent process, which will mean that participants will be more informed and committed to their participation in a trial. They are unlikely to have unanticipated experiences or challenges while taking part, and more likely to understand how important their continued participation would be to the validity of the study outcomes. They are also more likely to be able to cope with the inconvenience and the sometimes side-effects they may experience during study participation.


Retention cannot be taken for granted, and it is good to plan for high retention rates in studies, to set goals and to monitor those retention goals over the course of a trial. In low- and middle-income country settings there may be additional challenges with retention that mean careful planning and monitoring are even more important. This information is key on decisions like sample size, recruitment rates required and other related information, usually spelled out in the protocol and statistical analysis plan. Attached is a template retention plan that could be used as the basis of a plan for almost any kind of research study. This is provided as an example of which components could be included in such a plan. It is a good idea to include this in the protocol, so that it can be reviewed and approved by the ethics committee approving and reviewing the research study.


Click here to download a Partitipant Retention Plan

template which you can adapt to your study. You can also visit our extensive Templates Library for other useful tools and templates. Please also let us know any comments or experiences you have found.