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Systematic Review

Key techniques

Use the following techniques when entering your search terms into databases to help you find relevant results:

Phrase searching 

Command "   "  vertical quote marks (be careful with copy and paste)
Use To enclose a short phrase
Effect The search will find the exact phrase
Example "mental health"


Command                *  asterisk (in most databases)      A few databases truncate word stems automatically.                                         
Use On the end of a base/root word
Effect The search will find all the different word endings
Example teen* finds teen, teens, teenage, teenager, teenagers, teenaged

Combining search terms


Use between search terms describing one concept to find results containing any of the alternative terms.

Using OR broadens your search. 

Example: "vitamin C" OR "ascorbic acid" will find records with one of these terms or both. 


Use to combine your concepts to only find results containing all the concepts.

Using AND narrows your search. 

Example: "pain management" AND "mobile applications": both terms must be present in the results.


Use to exclude terms from your search results. 

Using NOT narrows your search. 

Example: diabet* NOT gestational: records that have the term "gestational" are excluded from the results.

Combining operators

When using a combination of AND and OR operators in a search, enclose the alternative terms or synonyms in parentheses.

Example: ("vitamin c" OR "ascorbic acid") AND (cold OR flu OR influenza) 

See Search strings for more details.


Useful tools

PubVenn - explore PubMed using venn diagrams to find the relative size of the citation set for each term as well as how those sets interact. These diagrams may help users visualize how complex searches work.

Proximity searching

This search technique is occasionally helpful for finding relevant results:

Command N# (EBSCO)     ADJ# (OVID)     W/# (Scopus )          Note: # is a number
Use Links two terms together (where a relationship between the 2 elements is inferred)

The search will find the first term only if it is located within a maximum number of words of the second term.

The order of the terms is not important.


EBSCO: (diabet*) N6 (foot OR feet) 

OVID: (diabet*) ADJ6 (foot OR feet)

Scopus: (diabet*) W/6 (foot OR feet)

Proximity searching narrows your search 

  • It is especially good for searching keywords within full-texts
  • Not all databases are capable of proximity searching

Subject Headings

Most databases have subject headings (controlled vocabularies) assigned to articles to describe what the article is about.

  • Subject headings are database specific
  • Subject headings yield precise results
  • If you use subject headings, you will also need to include keywords for the concept in your search 
  • Check Scope notes for details on what the subject heading covers.
  • Consider whether to use Explode or the Subheading options where they are available

Examples of subject headings used in some key databases:

Databases Subject Headings
AMED AMED Thesaurus
Business Source Complete Business Thesaurus
PsycINFO PsycINFO Thesaurus; MeSh

MeSH (Medical Subject Headings)

MeSH is a well-known thesaurus of biomedical and health related information. 

You will find MeSH in the Medline database, via EBSCO, OVID, PubMed or Web of Science.

Search Mesh headings using MEDLINE (EBSCO)


Yale MeSH Analyzer

The Yale MeSH analyzer (by the Yale University Library) allows users creating a MeSH analysis grid using PubMed. Use it to identify new search terms and the missing articles from PubMed. 

Video tutorial: Yale MeSH Analyzer (7.39 mins). 

MEDLINE (OVID) Expand Term Finder

  • Search MeSH headings, Stedman's Medical Dictionary and related terms from the Unified Medical Language System (UMLS)
  • Explore MeSH headings: MeSH tree, used for terms, scope notes, subheadings
  • Expand Term Finder is only available for MEDLINE currently

OVID Search Builder

A new OVID search tool allows users to: 

  • combine keywords, MeSH headings and synonyms in one search
  • apply multiple keywords in a PICO format
  • use specialised filters to search for adverse events, special situations and clinical queries 
  • search across MEDLINE and Embase databases 

Video: Ovid Search Builder (9.28mins).

Ovid search builder


Snowballing refers to using the reference list and the citations of papers you have selected for your research to identify additional papers. It involves looking backwards and forwards to identify other papers that may be relevant. This allows capture of papers that may not appear in search results due to keywords or terminology not used or indexed in a database.

Backward snowballing

  • Use the final set of selected papers from your systematic searching
  • Scan the titles in the reference list in each paper
  • Examine those that may be useful using your inclusion/exclusion criteria
  • Add relevant papers to your list selected for review if they are not included already

Forward snowballing

  • Examine the papers citing each paper in your final set of papers
  • Use your inclusion/exclusion criteria to determine if they are useful - check the title, abstract, then full text 
  • Add the papers to your list selected for review if not included already


Find citations for the papers in ScopusGoogle Scholar or any other resources. Citation analysis tools are available in Scopus. 

Other tools may also be used for citation analysis (e.g. for finding themes, how articles cite each other, and gaps): 

  • Vosviewer
  • OVID Citation Analyzer
  • Microsoft Excel

Further reading

Ecar, M., da Silva, J. P. S., Amorim, N., Rodrigues, E. M., Basso, F., & Solda, T. G. (2020). Software Process Improvement Diagnostic: A Snowballing Systematic Literature Review. 2020 XLVI Latin American Computing Conference (CLEI), 156-164. 

Mourão, E., Kalinowski, M., Murta, L., Mendes, E., & Wohlin, C. (2017, 9-10 Nov. 2017). Investigating the use of a hybrid search strategy for systematic reviews. 2017 ACM/IEEE International Symposium on Empirical Software Engineering and Measurement (ESEM), 193-198. 

van Haastrecht, M., Sarhan, I., Yigit Ozkan, B., Brinkhuis, M., & Spruit, M. (2021). SYMBALS: A systematic review methodology blending active learning and snowballing. Frontiers in Research Metrics and Analytics, 6(33). 

Wohlin, C. (2014). Guidelines for snowballing in systematic literature studies and a replication in software engineering. EASE '14, May 13 - 14 2014, London, England, BC, United Kingdom.