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Systematic Reviews

Where to Find Studies?

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Searching studies is a critical part of conducting systematic reviews because the search has to be comprehensive. In order to make it as comprehensive as possible, it has to be done in several sources such as subject databases, databases that index systematic reviews and grey literature such as unpublished studies, conference proceedings, or thesis.

A large proportion of research is published in journals indexed in databases. Search by subject in the lIHEID library A-Z Databases.

For a more detailed list of databases by subject, see the Appendix I of the Guide to information retrieval for Campbell systematic reviews.

To avoid unplanned duplication of research and enable comparison of reported review methods from other systematic reviews, authors may search on databases indexing systematic reviews:

- Centre for Reviews and Dissemination (CRD)                
A research department that specialises in evidence synthesis, assembling and analysing data from multiple research studies to generate health policy relevant research.            

- Cochrane Library
Evidence based practice, including Cochrane Database of Systematic Reviews (CDSR), the central register of clinical trials from the Cochrane Collaboration research groups and the Database of Abstracts of Reviews of Effects. International. A leading resource for systematic reviews in health care. Full-text. Dates vary. 

- The Campbell Collaboration                
Promotes positive social and economic change through the production and use of systematic reviews and other evidence syntheses for evidence-based policy and practice (useful for international development and education studies).

- EPPI-Centre                
Part of the Institute of Education and is committed to informing policy and professional practice with sound evidence through systematic reviews and research use. Library of international development reviews.

- 3ie: This database provides policymakers and practitioners with easy access to systematic reviews that examine evidence on the effects of social and economic development interventions in low- and middle-income countries. The database provides summaries of the findings and methodologies of existing systematic reviews and protocols of ongoing studies. It also offers quality appraisal of the featured systematic reviews. 3ie has repositories of impact evaluations, systematic reviews and evidence gap maps.

- Prospero: international database of prospectively registered systematic reviews in health and social care.

Grey Literature refers to literature that has not been commercially or formally published, and can include working papers, dissertations and theses, conference proceedings, and official documents such as government reports.

Institutional Repository: Research produced by the Graduate Institute


Try these resources to find international theses:

- British Library e-theses online service (EThOS) – you will need to register to download theses.

- Hong Kong University Theses Online – primarily English, but some Chinese only theses.

- Networked Digital Library of Theses & Dissertations (NDLTD) – international, full-text theses from participating collections around the world.

- Open Dissertations (Open Access) – a collaboration between EBSCO and BiblioLabs, this connects you to Open Access Doctoral EDTs (Electronic Theses and Dissertations) accepted by American Universities from 1955 onwards in selected full-text.

- WorldCat – useful for identifying theses held in North American libraries (Select 'Advanced Search' then 'Content' and 'Thesis/dissertations'.)


Try these platforms to find other types of grey literature documents:

- Institutional Repository for Information Sharing (IRIS)                
IRIS is the World Health Organisation's Institutional Repository for Information Sharing. The database provides access to all World Health Organisation (WHO) publications.

- ELDIS
ELDIS provides free access to relevant, up-to-date and diverse research on international development issues. The database includes over 50,000 summaries and provides free links to full-text research and policy documents from over 8,000 publishers. Each document is selected by members of and editorial team. Eldis is hosted by the Institute of Development Studies based in the UK.

- Grey Matters: a practical search tool for evidence-based medicine                
The Canadian Agency for Drugs and Technologies in Health produces a variety of publications that range from comprehensive systematic reviews and economic evaluations. Find international coverage organised by topic, including:

  • health technology assessment agencies
  • health economics
  • clinical practice guidelines
  • free databases
  • statistics

- OpenGrey                
A European Open Access grey literature database covering science, technology, biomedical science, economics, social science and humanities. Records are in English and content includes: technical and research reports, dissertations, conference papers, official publications and more. It has been discontinued in 2020 but it is now archived as a database in DANS Easy, a data archive.


Other suggestions:

Google and Google Scholar are useful for finding free reports and documents for example from government departments or universities.You can find a lot of grey literature with a well-designed Advanced Google search. Limit to site .gov and/or .edu for more reliable resources.


For a more detailed list of sources of grey literauture, see Appendix I of the following article:

Paez, A. "Gray Literature: An Important Resource in Systematic Reviews". Journal of Evidence-Based Medicine 10, no 3 (2017): 233‑40. https://doi.org/10.1111/jebm.12266.

The references cited in existing systematic reviews and meta analyses and reference lists of identified studies may also be searched for additional studies when doing a systematic review. A recent study found that 7% of the included studies in a social science systematic review were located through citation searching alone.

Citation searching used to be limited to using the indexes Science Citation Index Expanded, Social Sciences Citation Index, and Arts & Humanities Citation Index (via the Web of Science), but other resources including CINAHL, PsycINFO and Google Scholar, now include cited references in their records so these are also available for citation searching.

It is also important to identify ongoing studies, so that when a review is later updated these can be assessed for possible inclusion. Unfortunately, no centralised register of ongoing trials exists. Efforts have been made in the medical sciences to provide central access to ongoing trials and in some cases trial results on completion, either on a national or international basis. 

Databases include Clinicaltrials.gov and the EU Clinical Trials Register.

Campbell authors whose reviews concern or border on health related topics or outcomes, may find relevant studies in Social Care Online  or the World Health Organization (WHO) International Clinical Trials Registry.

The Directory of Open Access Repositories (OpenDOAR) and the Register of Open Access Repositories (ROAR) are comprehensive directories of academic open access repositories, providing both repository lists, as well as the possibility to search for repositories or search repository contents.

Google, Bing, Yahoo: Keywords such as ”study” or ”studies” or ”control group” may be used to limit the results to empirical research. Google Scholar may also be used; however this database is a compilation of records pulled from the subject databases (and other sources). Hence searching Google Scholar may produce a number of hits that have already been identified by the database searches.

Generally web searches are completed towards the end of the search phase of a review to ensure picking up the most current information.

Constructing the Search Strategy

Steps to build your research strategy:

  • identify key terms by breaking down your review question into concepts. Using the Population, Intervention, Comparison and Outcomes of PICO framework can help you; nevertheless is not essential that every element is used. For example, is better not to use terms for outcomes as databases can fail to retrieve articles if the outcome is not mentioned prominently in the record.
  • Find synonyms for every term and subject headings to use in every database. Subject headings are terms included in a controlled vocabulary (thesaurus). Thesaurus searching can be a narrowing search strategy but it is a quick way to retrieve papers on a topic.With keyword searching you will find more results as it broadens your search. It can be more effective than thesaurus for very specialised or emerging topics.
  • Find existing systematic reviews on any component of your topic and review search strategies included in the methods or appendix
  • Use logic operators to combine your terms:
    • "AND" narrows the search
    • "OR" broadens the search 
    • "NOT" excludes terms
    • "*" finds all forms of a particular word e.g. Child* covers child, children, childhood
    • parentheses ensure all terms will be searched together as a set 
    • quotations around a phrase searches that exact phrase

A good systematic search will comprise searching of both thesaurus terms and terms in the title and abstract fields and other appropriate fields that may be available.

Every database has its own characteristics and vocabulary that is why is important to know their particularities to optimise your search request. See this example of tutorials that can help you build your request in PubMed: