There are three approaches regarding choices for inclusion of studies based on their design in JBI systematic reviews. The first option is to clearly state in the protocol what study designs will be included (for example RCTs), and include only studies that are of this design in the review. This approach is transparent and at low risk of subjectivity during selection of studies. However, it runs the risk of leading to an empty review or a review with few included studies.
The second option is to consider using the hierarchy of study designs for including and excluding studies in the review. In this approach, authors may include other study designs if their preferential study designs are not located. If this is the case, there should be a statement about the primary study design of interest and the other types of studies that will be considered if primary study design of interest is not found. It is common to provide a statement that RCTs will be sought, and that in the absence of RCTs, other study designs will be included, such as quasi-experimental studies and observational studies. This is a pragmatic approach with the aim to include the best available evidence within a review.
The third option is to simply include all quantitative study designs (or all study designs up to a point of the hierarchy of evidence - for example experimental studies and cohort studies, both prospective and retrospective).. This inclusive approach is acceptable as it allows for examination of the totality of empirical evidence and may provide invaluable insights regarding the agreement or disagreement of the results from different study designs. Where feasible, JBI prefers and suggests reviewers consider option 3, the most inclusive approach. However, for many topics, this will present a great deal of information which may not be of use to best inform effectiveness.