Searching for Studies

Systematic literature searching is a critical component of the systematic review process. It involves a systematic search for studies and transparent reporting of study identification and how the findings of the review fit with the relevant evidence (Cooper et al., 2018).

Reviewers decide what kinds of studies can provide the types of evidence that is needed to address the central review question(s). The review question(s) determines the types of study designs that will be included and excluded.

A systematic review literature search should include a search of multiple databases, a search for grey (or gray) literature, personal communications with experts in the field, as well as a handsearch of high impact journals in the related field. Searching multiple databases and other sources makes a search more comprehensive and helps avoid publication bias (such as geographic bias of published studies or bias against publication of any negative results).

Consult a librarian to select subject-specific databases that index literature related to the topic of the systematic review. The Cochrane Collaboration and the Institute of Medicine (currently known as the National Academy of Medicine) recommend that the literature search be conducted by librarians or persons with extensive literature search experience. A 2015 study published in the Journal of Clinical Epidemiology shows that librarian involvement in systematic reviews improves both the quality and the reproducibility of the literature search.

In addition to database searching:

• Look at alternate sources of information including conference abstracts, technical reports, theses and dissertations, and grey literature sources to complement the database search retrieval.
• Search trial registries to identify ongoing studies for possible inclusion in the review.
• Review reference lists of existing reviews and relevant studies for additional studies.
• Hand search the table of contents of key journals to identify newly published/nonindexed publications.

Factors to consider in planning a search:

• Geographic considerations
• Publication language
• Publication dates 
• Relevance of data from unpublished sources

Adapted from: NIH Literature Search: Databases and Gray Literature

Types of resources to search for a systematic review:

• Systematic review and clinical trial databases
• PROSPERO
• Standard bibliographic databases (Medline, CINAHL PubMed, etc.)
• Grey literature, including conference proceedings and theses
• Reference lists from the most relevant articles
• Publication lists for the most important authors in the field
• Conversations with experts in the field

• Cochrane Handbook Chapter 4: Searching for and Selecting Studies -  Appendix of Resources for Publication  A list of resources for review authors. Provided to support and complement the Technical supplement to Chapter 4: Searching for and selecting studies.

References

Cooper, C., Booth, A., Varley-Campbell, J. Britten, N. & Garside, R. (2018). Defining the process to literature searching in systematic reviews: A literature review of guidance and supporting studies. BMC Medical Research Methodology 18, 85 (2018). https://doi.org/10.1186/s12874-018-0545-3

NIH Library. (n.d.). Literature Search: Databases and Gray Literature. Retrieved August 3, 2023, from https://www.nihlibrary.nih.gov/services/systematic-review-service/literature-search-databases-and-gray-literature

• Searching For Studies: A Guide to Information Retrieval for Campbell Systematic Reviews This guide presents key considerations on the information retrieval process and provides examples of strategies and resources for review authors and Trial Search coordinators (TSC) to reference in the planning and conduct of Campbell Systematic Reviews. 
• Cochrane Handbook Chapter 4: Searching for and Selecting Studies This chapter outlines some general issues in searching for studies; describes the main sources of potential studies; and discusses how to plan the search process, design and carry out search strategies, manage references found during the search process, correctly document the search process and select studies from the search results.