Articles have been assigned to one of the following categories based on the type of recruitment evidence.
Evaluation: The formal assessment of the effectiveness of a recruitment strategy through a nested randomised control trial, a case control or a systematic review.
Application: Information on the use of a recruitment strategy without formal evaluation. This includes informal evaluations such as level of recruitment before and after a strategy is applied and hypothetical models of recruitment 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.