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Evidence Synthesis Methodologies in the Health Sciences

Assemble Your Team

An evidence synthesis project should have:

  • A principal investigator.
  • A minimum of 2 reviewers who possess subject matter expertise.
  • A statistician, if a meta-analysis is planned.

Because evidence synthesis is based on a comprehensive search of relevant literature, the team should collaborate with a librarian who can provide expertise in search strategy design. HBLS' Evidence Synthesis Service offers various levels of collaboration with expert librarians. Please view the ESS guide for more details and to submit an intake form about your project.

Consider Your Timeline

It is not unusual for evidence synthesis projects to take significant time to complete. A study in BMJ Open analyzed 195 systematic reviews with protocols registered in PROSPERO. The authors determined that the mean time to completion and publication was 67.3 weeks (Borah et al., 2017). Consider how much time each team member has available, their level of experience, and the ratio of screeners to anticipated number of search results when planning your project.

Choose a Review Type

While systematic reviews are the most recognized type of evidence synthesis, there are many others. We highly recommend the following two articles as foundational reading:

Right Review is a helpful interactive tool to help you select an appropriate evidence synthesis type.

View the tabs in this box to learn about common types of evidence synthesis, with brief descriptions from the Grant & Booth article and links to relevant methodological guidance.

"The best known type of review, a systematic review seeks to systematically search for, appraise and synthesi[ze] research evidence." (Grant & Booth, 2009, p. 102)

Quoted from Grant & Booth, 2009, p. 95:

Methods used:

  • Search: Aims for exhaustive, comprehensive searching.
  • Appraisal: Quality assessment may determine inclusion/exclusion.
  • Synthesis: Typically narrative with tabular accompaniment.
  • Analysis: What is known; recommendations for practice. What remains unknown; uncertainty around findings, recommendations for future research.

Methodological guidance:

"Technique that statistically combines the results of quantitative studies to provide a more precise effect of the results."  (Grant & Booth, 2009, p. 94)

Quoted from Grant & Booth, 2009, p. 94:

Methods used:

  • Search: Aims for exhaustive, comprehensive searching; May use funnel plot to assess completeness.
  • Appraisal: Quality assessment may determine inclusion/exclusion and/or sensitivity analyses.
  • Synthesis: Graphical and tabular with narrative commentary.
  • Analysis: Numerical analysis of measures of effect assuming absence of heterogeneity.

Methodological guidance:

"Specifically refers to review compiling evidence from multiple reviews into one accessible and usable document. Focuses on broad condition or problem for which there are competing interventions and highlights reviews that address these interventions and their results." (Grant & Booth, 2009, p. 95)

Quoted from Grant & Booth, 2009, p. 95:

Methods used:

  • Search: Identification of component reviews, but no search for primary studies.
  • Appraisal: Quality assessment of studies within component reviews and/or of reviews themselves.
  • Synthesis: Graphical and tabular with narrative commentary.
  • Analysis: What is known; recommendations for practice. What remains unknown; recommendations for future research.

Methodological guidance:

"Preliminary assessment of potential size and scope of available research literature. Aims to identify nature and extent of research evidence (usually including ongoing research)." (Grant & Booth, 2009, p. 95)

Quoted from Grant & Booth, 2009, p. 95:

Methods used:

  • Search: Completeness of searching determined by time/scope constraints. May include research in progress.
  • Appraisal: No formal quality assessment.
  • Synthesis: Typically tabular with some narrative commentary.
  • Analysis: Characterizes quantity and quality of literature, perhaps by study design and other key features. Attempts to specify a viable review.

Methodological guidance:

"Assessment of what is already known about a policy or practice issue, by using systematic review methods to search and critically appraise existing research." (Grant & Booth, 2009, p. 95)

Quoted from Grant & Booth, 2009, p. 95:

Methods used:

  • Search: Completeness of searching determined by time constraints.
  • Appraisal: Time-limited formal quality assessment.
  • Synthesis: Typically narrative and tabular.
  • Analysis: Quantities of literature and overall quality/direction of effect of literature.

Methodological guidance:

"Attempt to include elements of systematic review process while stopping short of systematic review. Typically conducted as postgraduate student assignment." (Grant & Booth, 2009, p. 95)

Quoted from Grant & Booth, 2009, p. 95:

Methods used:

  • Search: May or may not include comprehensive searching.
  • Appraisal: May or may not include quality assessment.
  • Synthesis: Typically narrative with tabular accompaniment.
  • Analysis: What is known; uncertainty around findings; limitations of methodology.

"Generic term: published materials that provide examination of recent or current literature. Can cover wide range of subjects at various levels of completeness and comprehensiveness. May include research findings." (Grant & Booth, 2009, p. 94)

Quoted from Grant & Booth, 2009, p. 94:

Methods used:

  • Search: May or may not include comprehensive searching.
  • Appraisal: May or may not  include quality assessment.
  • Synthesis: Typically narrative.
  • Analysis: Analysis may be chronological, conceptual, thematic, etc..

Define Your Research Question

There are dozens of frameworks that can be used to define your research question. Some common ones can be found in the tabs in this box, borrowed from the excellent presentation on Frameworks and Eligibility Criteria from the Evidence Synthesis Institute.

We also recommend these two sources for additional question frameworks and examples:

Typically used for quantitative clinical studies.

PICO question framework
P Patient or Population
I Intervention
C Comparison
O Outcomes

A variety of PICO variations exist, including:

PICO variations
PIO - Comparison
PICOS + Study Type
PECO

- Intervention

+ Exposure
PICOT + Timeframe

Example:

Is the amount of environmental tobacco smoke exposure in infancy as measured by infant urinary and hair cotinine levels, associated with the occurrence of childhood cancer by age 14?
Patient or Population Infant
Intervention Environmental tobacco smoke exposure
Comparison No tobacco smoke exposure
Outcomes Childhood cancer by age 14

Qualitative, experience evaluation.

PEO
P Population
E Exposure
O Outcomes (or themes)

Example:

In adults aged 60 to 75, is there an association between community-dining and reduced loneliness?
Population Adults aged 60-75
Exposure Community dining
Outcome or themes Loneliness

 

Used in scoping reviews.

PCC
P Population
C Concept
C Context

Example:

What strategies and measures are used to rapidly share information about severe weather or fire conditions in rural communities?
Population Individuals living, attending school, or working in rural communities
Concept Communication about severe weather or fire
Context Rural communities

 

Mixed methods.

SPIDER
S Sample
P I Phenomenon of Interest
D Design of study
E Evaluation
R Research type

Example:

What are high school students’ perceptions and experiences of online-only instruction (e.g., over Zoom)?
Sample High school students
Phenomenon of Interest Online instruction
Design of study Questionnaire, survey, interview, case study, etc.
Evaluation Experiences and perceptions
Research type Qualitative or mixed method

 

Qualitative, outcome evaluation

SPICE
S Setting
P Perspective
I Intervention/exposure
C Comparison
E Evaluation

Example:

For small-scale agriculture farmers in India, what are the benefits of participating in cooperative groups compared to receiving a one-time financial subsidy?
Settings India
Perspective Small-scale agricultural farmers
Intervention/Exposure Participation in cooperative groups
Comparison One-time financial subsidy
Evaluation Benefits

 

Qualitative, health policy.

ECLIPSE
E Expectation
C Client
L Location
I Impact
P Professionals
S Service

Example:

How can university administrators reduce food insecurity on college campuses?
Expectation Food security
Client Students, staff and faculty
Location University or college campuses
Impact Reduction in food insecurity
Professionals Administration, food service providers
Service Food services, pantries, etc.

 

Consider Inclusion/Exclusion Criteria

Your inclusion and exclusion criteria work in tandem with your research question to determine the scope of your project. They may fall into areas such as exposure of interest, participants, study design, and geographic region, among others. This infographic from the University of Melbourne Library introduces many common criteria.

Check for Existing Reviews

Evidence synthesis is a huge commitment, so it's wise to determine whether other reviews have been completed or if protocols have been registered for projects currently in progress. We recommend the following places to search. You may also wish to search some of the recommended places to register or post your protocol in the Write and Register a Protocol page of this guide.

If there is a recent review published or registered on your topic, consider whether (a) there is enough newly published literature or major changes in the field to justify a new or updated review, (b) the existing reviews investigate different primary outcomes or have different inclusion/exclusion criteria, or (c) your research question can be revised to address a related but not identical topic.