Systematic Review and Evidence Synthesis

Grey (or gray) literature is literature produced by individuals or organizations outside of commercial and/or academic publishers. This can include information such as government reports, conference proceedings, graduate dissertations, unpublished clinical trials, and much more. The sources you select will be informed by your research question and field of study, but should likely include, at a minimum, theses and dissertations.

The intent of an evidence synthesis is to synthesize all available evidence that is applicable to your research question. There is a strong bias in scientific publishing toward publishing studies that show some sort of significant effect. Meanwhile, many studies and trials that show no effect end up going unpublished. But knowing that an intervention had no effect is just as important as knowing that it did have an effect when it comes to making decisions for practice and policy-making. While not peer-reviewed, grey literature represents a valuable body of information that is critical to consider when synthesizing and evaluating all available evidence.

Finding grey literature and searching it systematically is challenging. But there are a few approaches that you can take to add some structure to your search of this type of information:

• Refer to grey literature sources used for related evidence syntheses. Refer to both published evidence syntheses and registered protocols.

• Ask experts in the field for relevant grey literature sources. If you are an expert, include important grey literature sources, and ask colleagues for their recommendations.

• Search databases that specialize in grey literature.

  • This Preprint Repository Syntax Table can help you to identify potential databases to search, and aid you in successfully constructing a search strategy

• Search for theses and dissertations: There are a number of databases dedicated to theses and dissertations, which you can search using your search terms.

• Search clinical trials: There may be clinical trials being conducted that are relevant to your research question, but that haven't been published yet or never were published.

• Identify government agencies and international and non-governmental organizations that might publish technical papers and reports on your topic. Search their websites or any online libraries that they may provide. 

• Search conference proceedings and newsletters: Identify professional organizations or conferences at which researchers might be presenting work related to your topic. Search those conference proceedings or newsletters on the organization's website or by contacting organizational boards for access to past proceedings that may not be online.

• Contact known researchers in the field about whether there are any relevant ongoing or unpublished studies.

• Search professional and trade magazines. Professional magazines contain literature that is written by professionals in the field for other professionals in the field, but that may not be about research. Trade magazines contain advertisements and news very specific to a topic or industry.

Clinical Trials

Clinical trials may go unreported in the published literature. One useful method to identify unpublished clinical trials is to search clinical trials registries. The results may be available within the registries or you may need to contact the researchers associated with the trial for further information.

• ClinicalTrials.gov

• International Clinical Trials Registry Platform (WHO)

• Clinical Trials Registers

Conference Proceedings

This list of conference proceedings is not comprehensive, but are ideas of where to search. We recommend that you search proceedings for the three most relevant conferences for your evidence synthesis.

• OCLC PapersFirst
  OCLC PapersFirst is an OCLC index of papers presented at conferences worldwide.

Dissertations and Theses

Dissertations and theses are potentially rich sources of grey literature and are also usually considered grey literature themselves.

• Dissertations & Theses - How to find resources by format

Grey Literature Databases

• WHO Library Database

  • The WHO library database includes governing documents, reports and technical documentation.

Preprint Repositories

Preprint repositories are free online archives that allow researchers to identify studies in rapidly developing fields and can be used to identify studies that have not gone through the more formalized and traditional peer review process​. Search and export functionality varies from site to site.

• Preprints in Europe PMC

  • Includes bioRxiv, Research Square, Preprints.org, MedRxiv, ChemRxiv, PeerJ Preprints, F1000Res, SSRN, and others. Easy to export the basic citation to citation management software.

• MedRxiv

  • Free online archive of health sciences preprints (unpublished manuscripts) including original research articles, systematic reviews and meta-analyses, data articles, and articles describing methodological research/investigations and clinical research design protocols.

• OSFPreprints: Preprint Archive Search

  • Aggregated search that covers preprint servers using the OSF infrastructure, e.g., AfricArXiv, AgriRxiv, Arabixiv, EcoEvoRxiv, EdArXiv, engRxiv, Frenxiv, IndiaRxiv, NutriXiv, PsyArXiv, SocArXiv and others, in addition to other preprint sources, e.g., arXiv, bioRxiv, cogPrints.

• SSRN

  • "SSRN is a platform for the dissemination of early-stage research. From its initial focus on the social sciences in 1994, SSRN has grown to become the most interdisciplinary service of its kind, representing disciplines across the full research spectrum, including the applied sciences, health sciences, humanities, life sciences, physical sciences, and social sciences."

• arXiv

  • Open access to over 1.9M e-prints in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and systems science, and economics.

• bioRxiv: The Preprint Server for Biology

  • bioRxiv (bio-archive) is a free online archive and distribution service for unpublished preprints in the life sciences. Articles are not peer-reviewed, edited, or typeset before being posted online.

• engrXiv

  • Preprint archive for engineering articles.

• Research Preprints: Server List [Google Sheet]

Public Policy

• World Bank

  • ​The World Bank now makes all of their publications openly available online.

• WHO Institutional Repository for Information Sharing (IRIS)

  • Institutional WHO database of intergovernmental policy documents and technical reports. Can search by IRIS by region (Africa, Americas, Eastern Mediterranean, Europe, South-East Asia, Western Pacific).

• Health Research Web

  • A wiki with descriptors of national health research systems, ethics review committees, local and regional policies and research priorities and more.

Other Resources

• Grey Matters: A practical search tool for evidence-based medicine

  • This online manual provides a thorough list of sources for grey literature in medicine and a helpful checklist to help systematize your process.

• Searching the grey literature: A handbook for searching reports, working papers, and other unpublished research

  • Searching the Grey Literature is for librarians and information professionals interested in learning more about grey literature. This book will aid with crafting a grey lit search successfully, from start to finish. Many types of librarians will find the content of this book useful, particularly those in health or social science.

• Duke University Medical Center Guide to Resource for Searching the Grey Literature

  • Resources for trial registries, pharmacological studies, conference abstracts, government documents and more.

• Gray Literature Resources for Agriculture Evidence Syntheses

  • This is a comprehensive list of agricultural grey literature sources, one of several evidence synthesis worksheets and tools provided to help in non-medical evidence synthesis development and implementation.

• Searching for studies: A guide to information retrieval for Campbell Systematic Reviews

  • This document gives comprehensive guidance in searching for evidence syntheses, and offers a robust list of grey literature sources for social science disciplines.