Evidence Synthesis

Evidence synthesis is a type of secondary research that seeks to discover, evaluate, and analyze all available evidence on a topic. It uses "formal, explicit, rigorous methods to bring together the findings of studies already completed and to provide an account of the totality of what is known from that pre-existing research" (Gough et al., 2020). 

There are many different types of evidence synthesis. These can be roughly sorted from less formal to more formal. The graphic below sorts a selection of popular review types on a spectrum (Foster, 2024). 

Systematic reviews are a very well-known review type and when done correctly, result in a high level of evidence. In health and medicine, systematic reviews are often used to create or update clinical practice guidelines. However, not every evidence synthesis project is or should be a systematic review, and another type of evidence synthesis method may be better to address your particular topic and question. When getting started, it's helpful to at least have a passing familiarity with different review types. Use the following two resources to learn more about the different types of reviews and begin thinking about which one might be correct for your project. 

References:

Foster, M. J. (2024, June 5). Piecing together systematic reviews phase 1: Proposal [PowerPoint slides]. Network of the National Library of Medicine. https://www.nnlm.gov/training/class/piecing-together-systematic-reviews-phase-1-proposal 

Gough, D., Davies, P., Jamtvedt, G., Langlois, E., Littell, J., Lotfi, T., Masset, E., Merlin, T., Pullin, A. S., Ritskes-Hoitinga, M., Røttingen, J., Sena, E., Stewart, R., Tovey, D., White, H., Yost, J., Lund, H., & Grimshaw, J. (2020). Evidence synthesis ESI: Position statement. Systematic Reviews, 9, 1-9. https://doi.org/10.1186/s13643-020-01415-5

One thing that distinguishes formal evidence synthesis from more "traditional" reviews in the box above is that they require teams of multiple people. A true formal evidence synthesis (including but not limited to systematic, scoping, and rapid reviews) must have at least:

1. Reviewer #1 to screen articles 
2. Reviewer #2 to screen the same set of articles 
3. Principal investigator to resolve any conflicts 

And ideally: 

4. A librarian or search expert to write the search string - at FSU, full-time faculty have this option; refer to Evidence Synthesis Services link below for more information
5. A statistician if performing a meta-analysis

And optionally:

6. Other reviewers, especially if there are a lot of articles to screen; you may randomly split articles into batches as long as each is reviewed by two separate people 

If you'll be writing a review by yourself, you may want to call it a literature review or narrative review. You can still go through the searching, screening, evaluation, and data extraction steps on your own, but a formal evidence synthesis project requires multiple people to perform all the steps. Literature and narrative reviews can still be valuable contributions to the body of knowledge in your field, so this statement is not meant to discourage you from writing one. It's just to help you understand the requirements for formal evidence synthesis so you can plan your review accordingly.