Oxford LibGuides: Systematic Reviews and Evidence Syntheses: Searching for studies

Searching for systematic reviews

Once you have defined your question you can start the searching process. The first step in searching for studies is to locate previously conducted systematic reviews in your area of interest. This has three main purposes:

To verify that your question hasn't already been answered
To verify that there are no other review protocols registered with researchers already looking at the same question
To identify related systematic reviews that will need to be accessed so that you can review the reference lists for relevant primary studies.

Useful databases for identifying systematic reviews

Cochrane Database of Systematic Reviews (CDSR)
Available via the Cochrane Library.
Campbell Library
Available via the Campbell Collaboration.
PROSPERO: International prospective register of systematic reviews
Most authors will register their protocols here, so you can search the register for any in-progress reviews on your topic.
PubMed
Search PubMed and filter the results for Article Type – Systematic Review

Searching for primary studies

After searching for systematic reviews, the next stage is to find other 'primary studies' e.g. trials, diagnostic accuracy studies, qualitative studies. The identification of all studies relevant to your question is made up of three broad stages:

Initial scoping searches to inform the development of the protocol and set parameters of the search.
Development of a structured search strategy that is run across multiple databases
An iterative process for finding further studies through hand search of relevant publications, reviewing reference lists, citation searching, contact with authors and location of on-going studies.

The guides below provide an introduction to locating studies for different types of systematic reviews (therapeutic, diagnostic, qualitative...). You can also contact your outreach librarian or subject librarian for help and advice.

Cochrane handbook for systematic reviews of interventions (Chapter 4: Searching for and selecting studies) [ebook] by Julian P. T. Higgins; James Thomas (Editors)
ISBN: 9781119536628
Publication Date: 2022
Version 6.3, 2022. Chapter 4 by Carol Lefebvre et al., on behalf of the Cochrane Information Retrieval Methods Group.
Cochrane handbook for systematic reviews of diagnostic test accuracy (Chapter 7: Searching for studies) [ebook] by de Vet HCW, Eisinga A, Riphagen II, Aertgeerts B, Pewsner D.
Publication Date: 2008
Version 0.4 [updated September 2008].

Doing Realist Research. (Chapter 9: Scoping and Searching to Support Realist Approaches) [ebook] by Emmel N, Greenhalgh J, Manzano A, Monaghan M, Dalkin S, (editors)
Online ISBN:9781526485472
Print ISBN:9781473977891
Publication Date: 2018
Chapter 9 by Booth A, Wright J, Briscoe S.

Databases to search for primary studies

The development of a structured search strategy that is run across multiple databases is a key stage in identifying primary studies for inclusion in a systematic review. These are some of the databases relevant to systematic reviews in health care. Additional or alternative databases should be searched depending on your question.

Consult your outreach or subject librarian for advice on relevant databases for your question.

These databases and more can be accessed via Databases A-Z.

CINAHL
Gain access to informative health literature with CINAHL, the authoritative resource for nursing and allied health professionals, students, educators and researchers. Offering complete coverage of English-language nursing journals and publications from the National League for Nursing and the American Nurses' Association, this database covers nursing, biomedicine, health sciences librarianship, consumer health and 17 allied health disciplines, and contains:

Indexing for over 1,200 nursing journals and publications dating back to 1982.
Over 250,000 records.
Abstracts for over 250 journals, including over 450 author-supplied abstracts.
Indexing for journals, books and book chapters, dissertations, selected conference proceedings, standards of practice, pamphlets, educational software packages and audiovisual material.
Internal subject thesaurus with over 7,000 terms—2,000 unique to CINAHL.
Cochrane Library
The Cochrane Library is an electronic publication designed to supply high quality evidence to inform people providing and receiving care, and those responsible for research, teaching, funding and administration at all levels. Use it to find systematic reviews and clinical trial reports including randomised controlled trials.
EMBASE: Excerpta Medica
Embase is a major biomedical and pharmaceutical database bringing together the literature in drug research, pharmacology, pharmaceutics, toxicology, clinical and experimental human medicine, health policy and management, public health, occupational health, environmental health, drug dependence and abuse, psychiatry, forensic medicine, and biomedical engineering/instrumentation. It indexes over 3,500 international journals, including many important journals from Europe and Asia not found in other biomedical databases. Coverage dates from 1974. Updated weekly. Approximately 375,000 records are added yearly.
MEDLINE (Ovid)
MEDLINE is the United States National Library of Medicine's (NLM) premier bibliographic database providing information from the following fields: Medicine; Nursing; Dentistry; Veterinary medicine; Allied health; Pre-clinical sciences.
The MEDLINE database is the electronic counterpart of Index Medicus,Index to Dental Literature, and the International Nursing Index.
The Ovid MEDLINE database contains bibliographic citations and author abstracts from more than 4,600 biomedical journals published in the United States and in 70 other countries. The database contains well over 13 million citations dating back to 1950, including more than 130,000 population-related journal citations (unique to the former POPLINE database) that were added to MEDLINE in October of 2002. Although coverage is worldwide, most records are derived from English-language sources or have English abstracts. Abstracts are included for more than 75% of the records.
PsycINFO
PsycINFO provides citations with abstracts to the scholarly literature in the psychological, social, behavioural, and health sciences. The database includes material of relevance to psychologists, students, and professionals in related fields such as psychiatry, management, business, and education, social science, neuroscience, law, medicine, and social work. PsycINFO contains more than 2 million records spanning 1806 to the present. Journal coverage includes material selected from approximately 2,000 periodicals. Chapter and book coverage includes worldwide English-language material published from 1987 to the present; however, there is a substantial number of records covering books published earlier. Dissertations constitute approximately 12% of the database. APA is currently adding approximately 8,100,000 references annually through weekly updates.
Scopus
Scopus is a bibliographic database for science, medicine and some social sciences. It covers over 25,000 journals and over 300,000 books from over 7,000 publishers worldwide, providing access to over 90 million records going back as far as the 18th century.
Web of Science
Alternative names: WoK ; WoS ; Web of Knowledge. Provides access to and cross-searching of databases in the science, technology, engineering, mathematics, medicine, social sciences and humanities. It includes Science Citation Index, Social Sciences Citation Index, Arts and Humanities Citation Index, Data Citation Index. Also provides access to Journal Citation Reports for sciences and social sciences.

Reporting your search

You will need to document your search in order to comply with the Preferred Reporting Items for Systematic reviews and Meta-analyses (PRISMA).

This is what you need to record:

Databases searched – including version and dates of coverage
Summary of search terms used, plus full electronic strategy for at least one database
Any limits applied e.g. language, date, study type
Other search methods applied e.g. screening reference lists, forward citation searching, web searching and contact with authors

General tips on literature searching

Systematic searching: Practical ideas for improving results [ebook] by Paul Levay; Jenny Craven [Editors]
ISBN: 1783303751
Publication Date: 2019
Searching skills toolkit: Finding the evidence [ebook] by Caroline De Brún; Nicola Pearce-Smith
ISBN: 9781118463123
Publication Date: 2013

A systematic approach to searching: An efficient and complete method to develop literature searches
The article describes methods that can be used to create comprehensive search strategies for different databases.

Methodological search filters

Methodological search filters (or "hedges") are often applied to searches to retrieve study types that are relevant to answering the question posed in the systematic review.

For example, for systematic reviews of therapeutic interventions it's customary to restrict the search to randomised controlled trials. The guides below have information on methodological search filters.

Please contact your outreach or subject librarian to check that you're using the most appropriate filters for your topic.

Cochrane handbook
Section 4.4.7 Search filters
Health Information Research Unit, McMaster University, Hedges Team
The focus of the Hedges Project is to investigate ways to develop and harness search filters ("hedges") that will improve retrieval of scientifically sound and clinically relevant study reports from large, general purpose, biomedical research bibliographic databases.
InterTASC
The InterTASC Information Specialists' Sub-Group Search Filter Resource is a collaborative venture to identify, assess and test search filters designed to retrieve research by study design or focus.