Successfully recruiting and retaining the pre-specified number of patients to time and target is a commonly encountered challenge that negatively impacts clinical trials. Consequently there is a drive in research aimed at addressing these problems. However, the volume of research in this area needs to be managed in an efficient way that enables trialists to access and use relevant research as well as allowing trials methodologists to identify any gaps in the research.

ORRCA aims to collate relevant methodological research on recruitment and retention in a searchable database.


ORRCA has been developed through the Trials Methodological Research Partnership (TMRP), in particular the Hub for Trials Methodological Research (HTMR) and the HRB Trials Methodology Research Network (HRB TMRN). Ongoing maintenance of the resource is supported by the Trial Conduct Working group within TMRP which facilitates networking and collaborative research in a number of areas including trial recruitment and retention.

Search strategy and review process

Two seperate search strategies have been developed to identify recruitment and retention literature. These are then adapted for use in Medline (Ovid), CINAHL (EBSCO), PsycINFO (EBSCO), Scopus, Web of Science Core Collection (SCI-expanded, SSCI, CPCI-S, CPCI-SSH, ESCI) and the Cochrane library (CENTRAL).  Updates are undertaken periodically. 

A team of volunteer reviewers from across the globe are involved in reviewing returned records. Following title and abstract screening, full texts are obtained for potentially relevant records. Eligible articles are then categorised according to all relevant domains in the respective recruitment and retention frameworks 

Eligible articles are also categorised according to the following types of evidence:

Randomised Evaluation – The formal assessment of the effectiveness of a recruitment strategy through a nested randomised control trial.

Non-randomised Evaluation - A comparative evaluation of a recruitment strategy such as a case control study or pre/post test. (NB this has only been applied from 2017 publications onwards)

Application - Information on the use of a recruitment strategy without comparative evaluation. This includes studies reportig the use of a strategy and hypothetical models of strategies.  

Observation - Concepts, observations or information that may be used to develop recruitment strategies. This includes lessons learnt from trials and reasons for participation/ non participation.

Additional information has been extracted from articles to allow users to search for articles based on the methods, outcomes and approach of the recruitment research, as well as the recruitment context (recruitment setting, gender, age, intervention type etc.)

Further information on the development process can be found in our publication.



We have completed updates with 2018-2019 papers and are currently reviewing the future of ORRCA/ ORRCA2. if the resource is continued then the scope will be revised to ensure more timely updates.  

Changes to the recruitment domains roadmap were applied during the 2017 update. A summary of changes are available along with the previous version.