Background: Evidence synthesis is a constantly evolving field. There is now a plethora of evidence synthesis approaches used across many different disciplines. Historically, there have been attempts to organize the types and methods of evidence synthesis in the form of classification systems, typologies, or taxonomies. This scoping review sought to identify all the available classification systems, typologies, or taxonomies; identify how they were developed; their characteristics; and the types of evidence syntheses included within them.
Objectives: The objective of this scoping review is to identify evidence synthesis types and previously proposed classification systems, typologies, or taxonomies that have guided evidence synthesis.
Methods: This review has been developed in consultation with evidence synthesis taxonomy initiative advisory group, which includes over 100 evidence synthesisers. Utilising JBI guidance for scoping reviews, a rigorous database search of MEDLINE (Ovid); Embase (Elsevier); CINAHL with Full Text (EBSCO); ERIC (EBSCO); Scopus (Elsevier); Compendex (Elsevier); JSTOR; and grey literature search has identified 292 evidence sources for inclusion, and they are currently undergoing extraction. This scoping review considered documents such as discussion papers, commentaries, books, editorials, manuals, handbooks, and formal guidance from major organizations.
Results: This review identified many different types of classification systems identified for categorizing evidence synthesis approaches. These systems varied in how they were formatted, their characteristics, the evidence synthesis approaches included, and how they have organized evidence synthesis types. Lastly, this scoping review identified differing terminology relating to evidence synthesis types being utilized across fields and how this could be perpetuating confusion and poor conduct of evidence synthesis.
Conclusions: The development of typologies, taxonomies, and classification systems of evidence synthesis have not as yet been conducted through a systematic process, nor with extensive stakeholder engagement from members of the academic community. They have also been criticized for being incomplete and lacking consistency in the terminology. Although many of the pre-existing categorisation systems serve as a useful reference point for researchers, a comprehensive, continuously updated, and community-approved evidence synthesis taxonomy is needed. Patient, public, and/or healthcare consumer involvement: Yes, through the ESTI advisory board (100+ members).