Evidence based synthesis can be accomplished through many different synthesis methodologies. Grant and Booth 2009 describe 14 different review types. Each of the review types focus on producing a different type of outcome for the research team. Some types have greater or lesser critical assessment, control for bias, and and guidelines for search.
Search strategies: How rigorously designed are the search strategies? How comprehensive is the search expected to be? Are short cuts allowed? Is the search strategy assessed or peer reviewed as part of the process?
Database selection: How many databases are searched as part of the process? How comprehensive is the search expected to be?
Time: How quickly does the review need to be completed?
Personnel: Is it a research team? Can it be completed by one or two people?
Research question: Is the research question formulated in such a way that it can be tested through the literature? Is it structured with a research question framework?
Grant and Booth (2009) created a framework for describing the key attributes of evidence synthesis methodologies. The key attributes identified were:
These key factors can be used to determine whether an evidence synthesis model is appropriate for the research goals or articulated research question of a synthesis project.
Cornell University Library has created a flow chart of the six most common evidence synthesis types. You can find the chart at Cornell Decision Tree of Evidence Synthesis Methodologies.
Note that the triage steps for determining which type of review starts from the research scope, the available personnel and time, and then focuses on the research questions and resource types that are needed. In other words, to make a decision about the synthesis methodology, the decision point should start not with "I am going to do a systematic review" and instead should start with "What review will fit the available resources and research goals for this project?"
Novice evidence synthesis researchers frequently make the mistake of starting with the review type rather than the needs of the research project for a given type of review.