Table 1 Key factors and considerations for selecting to conduct one review type over another

1. Review scope: Is the scope of the research question broad?

I. If the planned review has narrowly defined PICO elements, a de novo SR or an update of an SR may be the most appropriate review type. If the planned review has a broader scope that expands upon one of the PICO elements, an overview of SRs may be the most appropriate review type.

2. Review objective and analytic approach: Is quantitative combination of findings needed to provide a summary measure or to rank interventions?

II. If the planned review requires quantitative combination of findings through MAs, ITC, or NMAs to provide a summary measure or to rank interventions, a de novo SR or an update of an SR should be considered instead of an overview of SRs. In an overview, a narrative, not quantitative, synthesis should be conducted.

3. Relevance: Are there one or more relevant SRs available?

III. The quantity of relevant SRs available may rule out certain review types for the planned review. If there is no relevant SR, no overview or update could be conducted. If there is only one relevant SR, no overview should be conducted.

4. Methodological quality: Are the relevant SRs of sufficiently high quality in methodology?

IV. Ideally, only SRs of high quality in both methodology and reporting should be used for updates and, in many cases, for overviews. In the absence of high-quality SRs, replication in the form of a de novo SR may appropriate.

5. Reporting quality: Are the relevant SRs of sufficiently high quality in reporting?

6. Comprehensiveness: Are the relevant SRs comprehensive?

V. If relevant SRs are not comprehensive or outdated and need to be supplemented with additional primary studies that are available, an overview of SRs may not be best. Instead, an update of an SR—if there is a high-quality SR available—or a de novo SR—if there is no high-quality SR available—is recommended for consistency in the analytic approach at the study level.

7. Currency: Are the relevant SRs up to date?

8. New evidence: Are there additional relevant primary studies missing from the relevant SRs?

9. Discordance in results: Are the findings of the relevant SRs discordant for unknown reasons?

VI. If the results of two or more relevant SRs with matching PICO elements are discordant, and reasons for discordance cannot be reliably determined, a de novo SR may be needed.