Guide to literature searching

What is grey literature?

It is a catch-all term for material published non-commercially such as by government agencies, industry bodies, NGO's or academic institutions. It includes:

Grey literature is frequently high quality and up to date information. In some areas it may be the only information available. However it is not peer reviewed and should be evaluated with this in mind.

Why should I search grey literature?

• To be fully comprehensive in your search.
• To account for publication bias.
  • Published studies have been shown to show greater treatment effects than unpublished or grey studies (read more...). Identifying results from trials that have been completed, but not published, can counter publication bias.
• Immediacy.
  • Results may appear in grey literature sooner than in journals due to reduced publication lag. This is especially important in fast moving topics such as COVID related research
• When the body of formally published material is small.
• In qualitative research, commentaries or advocacy positions may be relevant.

How to search grey literature

There is no central database for grey literature such as Medline or Embase. Instead searches will need to target multiple sources. The following resources can help:

Preprint servers MedRxiv (biomedical), PsyArXiv (psychology and psychiatry) and BioRxiv (biological, including biochemistry, genetics, pharmacology).

EuropePMC is an open science platform that searches multiple sources including many preprint servers.

Filter for preprints using the left hand sidebar.

WHO's Clinical Trials Search Portal aggregates results from multiple trial registries.

ClinicalTrials.gov is a database of privately and publicly funded clinical studies conducted around the world maintained by the US National Library of Medicine.

Finding clinical trials, research registers and research results is a website listing multiple trial registries with information on search options for each.

Trip database is a search engine for biomedical and health material. Once searched look at the right hand sidebar to restrict results to guidelines, regulatory materials, blogs etc. 

Guidelines International Network  - large collection of international guidelines. Also has a website with useful resources for those developing guidelines.

International Health Technology Assessment database (IHTA) - access to information about health technology assessments from around the world.

Google Advanced Search. Use advanced search to restrict to particular domains eg .gov.au or file types eg .pdf.

Using Google for domain searching can be helpful if a sites internal searching capability is poor. Eg adding 'site:www.medrxiv.org' to a Google search will search within the MedRxiv site. More information on our Google for research guide.

OAIster searches institutional repositories and digital libraries. 

Grey Matters includes an extensive list of national and international HTA web sites, drug and device regulatory agencies, clinical trial registries, health economics resources and drug formulary web sites. It is produced by the Canadian Agency for Drugs and Technologies in Health.

Analysis & Policy Observatory (APO) collects grey literature on many public policy areas, including health, from government, agencies, regulators, research institutes, non-for-profits and think tanks. Australian focused.

Most grey literature sources do not have the functionality of databases. This can make it hard to manage recording searches and collecting references.

• Keep a log of searching activity (url's, dates, terms). This will make it easier to report
• Use consistent terms between sources.
• Use a browser extension for your reference manager to capture citations as you find them. These (eg Zotero's browser extension) may also track the url and access date of your activity.

Be aware that search engines can return different results over time and location, reducing transparency and replicability. Recording your activity mitigates this.

Assessing grey literature

Use the ACCODS checklist to assess the quality of grey information on these criteria: Authority, Accuracy, Coverage, Objectivity, Date, Significance.