Systematic Reviews- Legacy (2020-2022): Design and Conduct Literature Searches

The goal of a systematic search is to retrieve all results that are relevant to your topic. Because systematic review searches can be quite extensive and retrieve large numbers of results, an important aspect of systematic searching is limiting the number of irrelevant results that need to be screened. Librarians are experts trained in literature searching and systematic review methodology. Partnering with a librarian will save you time and improve the quality of your review.

Systematic review quality is highly dependent on the literature search(es) used to identify studies. To follow best practices for reporting search strategies, as well as increase reproducibility and transparency, you should document various elements of the literature search for your review. To make this process more clear, a statement and checklist for reporting literature searches has been developed and is available on PRISMA's website and can be found below.

At a minimum, certain elements should be documented and reported like where you searched like databases, including name (i.e., Scopus) and platform (i.e. Elsevier), websites, registries, and grey literature searched. In addition, this also may include citation searching and reaching out to experts in the field. Search strategies used in each database or source should be documented, along with any filters or limits, and dates searched. If a search has been updated or was built upon previous work, that should be noted as well. It is also helpful to document which search terms have been tested and decisions made for term inclusion or exclusion by the team. Last, any peer review process should be stated as well as the total number of records identified from each source and how deduplication was handled. 

If the search strategy for your systematic review is being created and run by your review team's librarian, the search documentation will be handled entirely by the librarian.

You can document search strategies in word processing software you are familiar with like Microsoft Word or Excel, or Google Docs or Sheets. A template, and separate example file, is provided below for convenience. 

*Some databases like PubMed are being continually updated with new technology and algorithms. This means that searches may retrieve different results than when originally run, even with the same filters, date limits, etc.

Systematic searches balance precision and sensitivity. Precise searches will retrieve some relevant results but have a high risk of missing relevant literature. Sensitive searches retrieve most of the relevant literature but will also retrieve a higher proportion of irrelevant results. As demonstrated by the image below, it is impossible to achieve high precision and high sensitivity at the same time. Librarians search in an iterative manner to refine results and achieve an ideal balance of precision and sensitivity.

Image reused from the University of Toronto Libraries

Use PICO or another question development framework to translate your research question into search terms. 

There are many different types of synonyms to consider including in systematic searches.

Terms with similar meaning:

• Early enteral re-feeding
• Early enteral nutrition

Terms that have different spellings:

• Re-feeding
• Refeeding

Acronyms:

• Necrotizing enterocolitis
• NEC

Concepts described inconsistently:

• Quality of life
• Satisfaction

Broad versus specific terms:

• Necrotizing enterocolitis
• Enterocolitis

Keywords versus controlled-vocabulary:

• Necrotizing enterocolitis
• "Enterocolitis, Necrotizing"[Mesh]

Discipline-specific jargon:

• Enteral nutrition
• Force feeding

There are several resources to consider when searching for synonyms. Scanning the search results of preliminary searches may help you identify additional search terms. Wikipedia, or other encyclopedias, may be helpful when looking for additional terms. Other databases can also help you identify synonyms. For example, PubChem can be used to look up drugs to find additional names and chemical compounds.

Controlled vocabulary is a set of terminology assigned to citations to describe the content of each article. Many databases assign controlled vocabulary terms to citations, but their naming schema is often specific to each database. For example, the controlled vocabulary system searchable via PubMed is MeSH, or Medical Subject Headings. More information on searching MeSH can be found here.

Searching with controlled vocabulary can improve the relevancy of search results. Not all citations are indexed with controlled vocabulary terms, however, so it is important to combine controlled vocabulary searches with text word searches. For example, combining controlled vocabulary and text words in PubMed would look like this: ("Enterocolitis, Necrotizing"[Mesh] or necrotizing enterocolitis[tw] OR NEC[tw])

As mentioned above, databases with controlled vocabulary often use their own unique system. A listing of controlled vocabulary systems by database is shown below.

Controlled vocabulary may be outdated and some database allow users to submit requests to update terminology.

Boolean operators are used to combine terms in literature searches. Searches are typically organized using the Boolean operators OR or AND. OR is used to combine search terms for the same concept (ie, refeeding). AND is used to combine different concepts (ie, refeeding and necrotizing enterocolitis). An example of how Boolean operators can affect search retrieval is shown below. Using AND to combine the two search terms will only retrieve results where both terms are used. Using OR to combine the two search terms will retrieve results that use either term separately or both together. It is important to note that, generally speaking, when you are performing a literature search you are only searching the title, abstract, keywords and other citation data. You are not searching the full-text of the articles.

Databases can be multidisciplinary or subject specific. Choose the best databases for your research question. Databases index various journals, so in order to be comprehensive, it is important to search multiple databases when conducting a systematic review. Consider searching databases with more diverse or global coverage (ie, Global Index Medicus) when appropriate. A list of popular databases is provided below. You can access UNC Libraries' full listing of databases on the HSL website (arranged alphabetically or by subject).

Databases often use their own set of terminology and syntax. When searching multiple databases, you need to adjust the search slightly to retrieve comparable results. Resources to help with this process are listed below.

Literature searches can be supplemented by hand searching. One of the most popular ways this is done with systematic reviews is by searching the reference list and citing articles of studies included in the review. Another method is manually browsing key journals in your field to make sure no relevant articles were missed. Other sources that may be considered for hand searching include: clinical trial registries, white papers and other reports, pharmaceutical or other corporate reports, conference proceedings, theses and dissertations, or professional association guidelines.

Grey literature typically refers to literature not published in a traditional manner and often not retrievable through large databases and other popular resources. Grey literature should be searched for inclusion in systematic reviews in order to reduce bias and increase thoroughness. There are several databases specific to grey literature that can be searched.

Search hedges are search strategies or filters that have been expertly developed, or at times validated, for sensitivity and precision. Search hedges can be incorporated into your search to save time and improve efficiency. Search hedges, also know as filters, are not the same as the general filters found in various databases. Search hedges should be used carefully as they are database specific.