A Guide to Evidence Synthesis: Types of Evidence Synthesis

Systematic reviews are the most commonly referred to type of evidence synthesis.  However, the phrase "systematic review" is commonly misused as a blanket term to describe review methodologies.  Although all review methodologies use a "systematic" approach, here is a list of qualities that distinguish a true systematic review.  

• Provides explicit research questions that identify participants, interventions, comparisons and outcomes (PICO).  
• Provide numbers of studies screened, assessed for eligibility, and include in the review the reasons for inclusion/exclusion at each stage, typically in the form of a flow chart. 
• Compares, evaluates, and synthesizes evidence in a search for the effect of an intervention
• Time intensive and can take months or years to complete.
• Requires a team (minimum of 2 reviewers) for blinded screening, data extraction and critical appraisal.

The following link provides a comprehensive list and details of the sections/topics found in a systematic review as well as a PRISMA Flow Chart Generator to showcase titles screened and related inclusion/exclusion criteria.

Scoping reviews are best designed to identify gaps in existing literature that is large, complex, or heterogeneous in nature which cannot fall under a systematic review.  This review allows for multiple research designs to be relevant, while a systematic review will only focus on studies that include interventions. 

The following link provides an article discussing and comparing a scoping review to systematic review.  Additionally, this article highlights a detailed framework for a scoping review. 

Often used for new or emerging research topics.  A rapid review speeds up the systematic review process by omitting stages of the systematic review making it less rigorous.  Literature searches are not as comprehensive and subject to increased limitations and potential biases. Typically this review can be completed in a 5 week period and may only include one reviewer. 

Focuses on the review of other systematic reviews on a particular topic.  This methodology is beneficial when competing interventions exist.  Umbrella reviews support a research question with a broader scope when compared to a research question for a systematic review. 

Meta-analysis combines findings from quantitative studies.  Statistical methods can be used to objectively evaluate, synthesize, and summarize results from multiple studies.  A meta-analysis can be conducted independently or part of a systematic review. 

A broad term referring to reviews with a wide scope and non-standardized methodology.  The search strategies, comprehensiveness and time range covered will vary and do not follow an established protocol.  Typically a literature review will seek to reach a level of "saturation" that describes the point where no new evidence/data can be found. Usually this process is not tracked or described within the finished study manuscript.