Per NIH Library (n.d.):
- A systematic review search includes a search of databases, gray literature, personal communications, and a handsearch of high impact journals in the related field. (See NIH Library list of recommended databases and gray literature sources)
- a comprehensive literature search can not be dependent on a single database, nor on bibliographic databases only.
- inclusion of multiple databases helps avoid publication bias (georaphic bias or bias against publication of negative results).
When searching for studies, customized search strategies must be constructed for each database, as terminology will vary across disciplines and databases. Questions to ask:
- What are the key concepts to be searched? Break research questions down into individual concepts to create search terms. Use the PICO format to construct your research question and identify the key concepts to be searched.
- How are these key concepts represented in each relevant (or related) field and across different cultures?
- What are the related terms for these key concepts? Think of alternative terms and concepts that may have potentially addressed the same research question.
- How are these key concepts represented in the controlled vocabulary within each database?
Authors may not describe their methods or objectives well and indexers are not always experts in the subject areas or methodological aspects of the articles that they are indexing. Available indexing terms might not correspond to the terms the searcher wishes to use. It is recommended that both keywords (natural language terms) and controlled vocabulary (or subject headings) be used in a search strategy.
To identify controlled vocabulary terms for a particular database, retrieve articles that meet the inclusion criteria and note common keywords and subject terms the indexers have applied to the articles. Identify controlled vocabulary terms using the thesaurus provided within each of the databases searched. In order to identify as many relevant records as possible searches should comprise a combination of subject terms selected from the controlled vocabulary or thesaurus with a wide range of keywords. Consider synonyms, related terms, variant spellings, and truncation. Questions to ask:
- Which terms should be searched as controlled vocabulary or as keywords (natural language)?
- What keywords should be truncated?
- Which Boolean operators (AND, OR, NOT) should be used and how should they be logically arranged?
- What limiting features are available to target primary studies only (for example, use of Document Type codes)?
- What time period should be searched?
- Will the search be limited by language?
Developing a search strategy is an iterative process. The included terms may be modified based on search results. A search strategy for a systematic review attempts to be as extensive as possible to ensure that most (or all) of the relevant studies are included in the review..It can be useful to start with a set of studies are already known to be are relevant. These studies can be used to test a search strategy by checking that the search results include the studies known to be relevant. It is more desirable to have a high sensitivity search (find as many articles as possible that may be relevant) rather than high specificity (making sure the articles retrieved are all relevant).so that you do not miss anything (Siddaway et al., 2019).
Helpful Hints
- Build a master search strategy in one database and tranlate the search into other relevant databases selected for inclusoion in the literature search.
- Use the PRESS 2015 checklist (Peer Review of Electronic Search Strategies) when formulating your search strategy for each database.
- Use controlled vocabulary specific to each database in addition to keywords. Consider synonyms, alternate spellings singular and plurals and word ending variations for your keywords.
- Keep detailed records of your search strategies and search results as you search.
- Search primary databases as well as grey literature (e.g., clinical trials registries, organization websites, preprints, etc.), cited and citing references
- Use the PRISMA-S extension to report your search strategy when writing your systematic review.
Keep records of the details of the searches and their results (Siddaway et al., 2019):
- Make a list of the number of studies excluded at the title/abstract screening stage
- Make a table to record individual studies that were excluded at the full text screening stage along with brief reasons for excluding each study based on your inclusion and exclusion criteria.This justifies the exclusion of studies that some readers might have expected to be included.
- Make a table that briefly describes efforts made to find and retrieve unpublished work.
- Make a table to describe in detail the characteristics of studies included in the review.
Search Tracking
Below is a handy search tracking spreadsheet adapted from MD Anderson Library to help you keep track of search strategies and results from each database searched:
References
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
Siddaway, A. P., Wood, A. M., & Hedges, L. V. (2019). How to do a systematic review: A best practice guide for conducting and reporting narrative reviews, meta-analyses, and meta-syntheses. Annual Review of Psychology, 70, 747–770.