Module 4: Searching for literature

Citation searching is conducted after the main search and the screening stage in a review. It involves using seed references, the included articles following the screening stage of the review, to find more relevant studies.

Citation searching is done in two ways: 

• Backward citation searching: looking at the references cited in the seed references.
• Forward citation searching: identifies articles that cite the seed references.

These methods complement database searching and are useful for topics that are difficult to define or use non-standardised language.

Citation databases can be used to search for seed references as a way of identifying both cited and citing references. Common citation databases include: Scopus, Web of Science and Google Scholar.

There are also citation mapping tools that identify citation relationships between papers. These tools can be useful to identify additional papers not found through traditional database searching. For example, ResearchRabbit.

Be sure to document and report any citation searching (including seed references and sources checked) as part of the search strategy.

For further information on citation searching check out the Tarcis paper.

Grey literature is material containing research or information relevant to a topic that is not commercially published. Searching grey literature can uncover material relevant to your review that isn't indexed in major databases and can help to minimise publication bias.

There are no specific guidelines for searching grey literature for reviews. However, CADTH have provided a grey literature checklist which contains advice to potential health-focused grey literature sources.

Different types of grey literature will suit various topics. Types of grey literature include:  

• Conference abstracts and papers
• Technical and other trade papers
• Dissertations and theses
• Government/organisational policies and reports
• Regulations and guidelines
• Statistical reports
• Unpublished systematic reviews and clinical trials
• Preprints and unpublished papers/manuscripts
• Datasets
• Bibliographies and documents in repositories

Search engines, Clearinghouses and other website searches are useful for locating conference proceedings, government, and industry reports. 

Trials registers can help to find unpublished clinical trials, such as ClinicialTrials.gov. There are also specialist databases containing grey literature, such as APA PsycExtra.

Be sure to document and report grey literature searches as part of the search strategy.

For more information on grey literature check out to the Grey literature guide.

Handsearching is used in systematic reviews. It involves page-by-page searching through key topic journals or conference publications for relevant studies that may not be indexed in the major databases or are only available in conference proceedings.

Databases such as Scopus or Web of Science can help you to identify key journals. After running your topic search, you can use features such as Refine Results or Analyze Results to find which journals publish the most articles relevant to your topic.

Be sure to document and record any handsearching as part of reporting the search strategy.

Check your review methodological guidelines for more specific instructions.

There may be relevant studies important to your review that are currently in progress and not yet registered or published. To identify these studies, some methodological guidelines recommend contacting key researchers or experts.

Experts can be found through:

• posting to relevant listservs (email lists) or professional organisations
• contacting authors of studies included in your review
• identifying known experts through conferences and papers. 

Be sure to document all the experts consulted for the review.

Check your review methodological guidelines for more specific instructions.

Conducting reviews can be time-consuming. Additional relevant studies may have been published since completing the searching phase of your review. 

The main search should be rerun in relevant databases before finalising the review to ensure inclusion of any additional relevant studies.