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Systematic Reviews & Other Syntheses

MEDLINE: Did you know?

You can search MEDLINE using PubMed or the Ovid interface. PubMed is a free resource that you will always be able to access, whereas Ovid MEDLINE is a proprietary resource that has been purchased by Queen's University Library. The Ovid interface allows adjacency searching (searching for terms within so many words of one another), which PubMed does not, and may be more user-friendly for new searchers than PubMed. 


Studies for your syntheses may be found in the form of:

  • journal articles
  • conference abstracts or proceedings
  • dissertations and theses
  • governmental or private sector research
  • ongoing or unpublished clinical trials

Deciding where to search will largely depend on the review type and topic. How many resources you search may depend on time constraints. Locating studies for syntheses often includes searching for both published and unpublished ("grey") literature.

Library Databases

Queen's University Library provides access to broad coverage databases in medicine and the health sciences (such as MEDLINE and Embase) as well as smaller subject-specific databases (such as PsycINFO and CINAHL). Depending on your review topic, it may also be appropriate to search in databases from other disciplines such as the social sciences, law or education.

Below is a list of the most popular health sciences databases used for syntheses that Queen's University Library provides access to. It is not a comprehensive list of potentially relevant databases. To discover more databases that might be relevant for your topic, try the Browse Databases by Subject feature on the library website or speak with a librarian.

Grey Literature Resources

What is grey literature?

Grey literature is the term used to describe literature that is not formally published as a book or journal article (Higgins, 2011), including:

  • conference proceedings
  • dissertations and theses
  • governmental or private sector research
  • ongoing or unpublished clinical trials

Related library guides:

In addition to the grey literature resources listed below, there are related library guides on: Finding Theses and Dissertations and Government Information.

Grey literature resources:

Some library databases contain grey literature in addition to journal articles. For example, PsycINFO includes dissertations; Embase and Medline/PubMed include conference proceedings. Library databases like Medline/Pubmed, however, do not provide comprehensive coverage of grey literature so other resources should be searched to identify literature that is not formally published.

Clinical trials registries:

For quantitative systematic reviews of healthcare interventions, it is strongly recommended to search clinical trial registries for on-going and unpublished trials in order to limit publication bias. Refer to Section 6.2.3 Unpublished and Ongoing Studies in the Cochrane Handbook for Systematic Reviews of Interventions for more information.


Hand-searching journals:

Hand-searching relevant journals page by page can be incredibly time-consuming but it may identify additional studies that have been missed by database search strategies or studies from journals that are not covered in electronic databases.

Research evidence:

  • A systematic review by Hopewell et al. (2007) on the efficacy of handsearching to identify reports of randomized trials found that "...a combination of handsearching and electronic searching is the most comprehensive approach in identifying reports of randomized trials" (p. 2). However, the authors noted "...where time and resources are limited, searching an electronic database using a complex search will identify the majority of trials published as full reports in English language journals, provided, of course, that the relevant journals have been indexed in the database" (p. 2).

Hand-searching conference proceedings:

Databases may not index conference proceedings, and even if they do, coverage may be limited. Hand-searching relevant conference proceedings that are available on association or conference websites may identify additional relevant studies.

Searching Reference Lists

Once you have identified eligible studies for your knowledge synthesis by searching the appropriate resources above, it is best practice to then check the reference lists of these studies to identify any additional studies. The reference lists of related reviews may also help to identify additional studies.

Research evidence:

  • A systematic review was conducted by Horsley et al. (2011) to determine whether searching reference lists actually identifies additional studies during the systematic review process. Most studies included in their systematic review reported that this practice did identify additional studies; however, the evidence supporting this practice most often came from case report data and not high quality research. Despite this, Horsley et al. still recommend that systematic reviewers screen the reference lists of relevant studies to find additional studies, especially when you're struggling to find relevant information.

Cited Reference Searching

Citation indexes such as Web of Science (or the search engine Google Scholar) can be used for cited reference searching. Citation reference searching allows you to identify where eligible studies have later been cited to see if this identifies any additional studies. For example:

If you search for a relevant study in Google Scholar you will see a "Cited by..." link underneath the study information that indicates how many times the study has been cited (unless the study has not yet been cited). You can follow this link to view where the study has been cited to see if this locates any additional studies.

For more information, refer to the section on Citation indexes in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins, 2011).


Higgins, J.P.T. & Green, S. (Eds.). (2011). Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 [updated March 2011]. The Cochrane Collaboration. Available from
Hopewell, S., Clarke, M., Lefebvre, C., & Scherer, R. (2007). Handsearching versus electronic searching to identify reports of randomized trials. Cochrane Database of Systematic Reviews, 2.

Horsley, T., Dingwall, O., & Sampson, M. (2011). Checking reference lists to find additional studies for systematic reviews. Cochrane Database of Systematic Reviews, 10(8).