Literature Search Overview | What We Need From You | What You Get From Us | Databases | Grey Literature | Terminology
Literature Search Overview
A core tenet of evidence synthesis reviews is reducing biases, and a comprehensive literature search is foundational to reducing bias. A comprehensive literature search cannot be dependent on a single database, nor on bibliographic databases (e.g., PubMed, Embase, Scopus, etc.) only.
Cochrane Collaboration and Institute of Medicine recommend that the literature search be conducted by librarians or persons with extensive literature search experience. NIH Librarians are expert searchers who are ready to partner with you on developing the best searches for your project, including documentation and methodological reporting of the searches in databases and grey literature.
Why should you include a librarian? Systematic reviews with librarian co-authors have higher quality reported search strategies (Rethlefsen et al., 2015). NIH Librarians who specialize in evidence synthesis are expert searchers and familiar with
- Following established guidance on comprehensive searching (e.g., Cochrane Handbook, JBI Manual)
- Consulting on search terms and related terminology
- Selecting appropriate databases and information sources
- Developing, testing, and optimizing comprehensive searches in multiple databases
- Peer reviewing search strategies created by NIH Librarians
- Identifying, discovering, and collecting grey literature
- Exporting search results into EndNote and Covidence
- Documenting the searches following best practices outlined by PRISMA for Searching
- Drafting the PRISMA Flow Diagram
- Writing the search methods for publication using the appropriate PRISMA reporting checklist
- Preparing the search strategies used as supplemental materials for publication
Contact us to learn more about how we can partner with you on a successful evidence synthesis review. Our Evidence Synthesis Service is only available to NIH and HHS staff with library access.
What We Need From You
After contacting us, we will schedule a consultation with you to discuss your review and our service. As part of this process, we ask you to fill out our checklist for information on your research question, relevant articles you may know of, inclusion/exclusion criteria, and other factors that might impact the search (e.g., keywords, limits, sources to search, etc.).
Authorship
Conducting an evidence synthesis requires significant time and expertise to do it correctly. This includes the work of the NIH Librarian. Given the expertise and significant time commitment involved in this work, NIH Librarians' contributions to reviews warrant co-authorship on manuscripts.
If you have questions about authorship, please contact the Evidence Synthesis Service.
Research question: PICO and other frameworks
Having a well-formed research question is key to a successful evidence synthesis review. There are many types of frameworks to pick from, depending on the type of review and subject area. One framework you may have heard of, PICO – Population, Intervention, Comparator, and Outcome(s) – works well for reviews of therapeutic effectiveness. For broader questions, such as those that a scoping review may answer, PCC (Population, Concept, Context) might work better. See our Start Here section on Refine the question to see more information on the types of framework you may want to use.
If you aren't sure which framework to use or need assistance with refining your research question, we can help! The NIH Librarian working with your group will ask questions to guide you in forming your research question. This process helps us with developing a comprehensive search for your question as well.
What You Get From Us
After receiving your draft manuscript, the NIH Librarian will:
- Provide the text for the methods and results of the searching and the final search strategies in publication-ready format
- Confirm the PRISMA Checklist was correctly used and cited
- Review the PRISMA Flow Diagram to ensure it includes the correct numbers
The searches will be in a reproducible table format to serve as supplemental materials or appendices for publication. Besides comprehensive searches tailored to your research question, we offer expert advice on your protocol and support for using Covidence.
Databases
For our NIH and HHS users, the NIH Library provides access to numerous bibliographic databases. Some of the databases we use for evidence synthesis searches include:
- PubMed which includes MEDLINE (recommended by Cochrane) (anyone can access)
- CINAHL Plus (NIH and HHS users)
- Cochrane Library, which includes Cochrane Central Register of Controlled Trials (CENTRAL; recommended by Cochrane) (NIH and HHS users)
- Embase (recommended by Cochrane) (NIH and HHS users)
- Global Index Medicus (anyone can access)
- PsycInfo (NIH and HHS users)
- Scopus (NIH and HHS users)
- Web of Science, which includes multiple databases (NIH and HHS users)
We may suggest other databases and resources based on your research question or topic area. If you are not from NIH or HHS, we are not able to provide access to these databases and you should contact your organization’s library for more information.
Grey Literature
Grey literature is anything produced outside of traditional commercial or academic publishing – basically anything outside of peer-reviewed journal articles found in bibliographic databases.
Including grey (the British spelling is generally preferred) literature in your evidence synthesis review helps reduce publication bias and include as much relevant evidence as possible. The Cochrane Handbook states that searching grey literature is "highly desirable"; the JBI Manual for Evidence Synthesis also recommends searching grey literature sources.
For some topics, including grey literature is critical as this is where researchers share their findings and information about their programs, projects, or studies. There are many types of grey literature, including:
- Preprints
- Clinical trial registries
- Theses and dissertations
- Conference abstracts, posters, presentations, or proceedings
- Patents
- Reports from governmental (GAO, CRS) or non-governmental organizations (e.g., WHO)
- Regulatory documents
- Policies and consensus guidelines
- White papers, technical reports, program evaluations
Terminology
The terms we use to describe populations, diseases, symptoms, and medical conditions evolve over time. Comprehensive literature searches should reflect these shifts. For that reason, it may be necessary at times for searches to include outdated terminology and common misspellings (Turner and Eisenstein, 2023) to be truly comprehensive. NIH Librarians collaborating with your review team will work with you to select the necessary terms, including potentially outdated ones, for a comprehensive search.