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After refining your clinical/research question, the next step is to identify related search terms and synonyms that represent each main concept. While doing this, be careful not to introduce irrelevant terms or go beyond the scope of the search question. An effective literature search combines both keywords and subject headings for each main concept.

Note: This is an iterative process, as you find relevant articles, you will discover additional search terms.. A chart or table can be useful to keep track of search terms. 

Is there a difference between keywords and subject headings? 

Yes!  Keywords are the natural language terms and phrases used to describe search concepts. Subject headings are "controlled language" words used by databases to ensure that all items relevant to a particular topic will be found. Searching with subject headings is a consistent and precise way to search many databases. However, it is not as flexible as a keyword search because you must know the exact term to use. 


Should l use keywords or subject headings in my search? 

Both! An effective literature search combines keywords and subject headings for each main concept.

Search Tip: Use keywords as a jumping off point to find corresponding and relevant subject headings for the databases that you are searching on. Remember to keep track of keywords and subject headings used in your search.

Keywords are the natural language terms that you type into a database, such as Medline, or a search engine, such as Google. Keywords are the terms found in places such as the title, abstract, and author provided keywords. The database will word-match your keywords against fields such as these and deliver results that match your keywords.

Not all authors use the same terminology, so try to think of the different ways that authors might express the concepts for which you are searching. This will ensure you capture all relevant articles. Some things to consider:  

Alternate spelling

pediatric or paediatric

Alternate ending

hospital or hospitalised or hospitalisation (or hospitalization)

Word form

physiotherapy or physio therapy

Plural form

child or children, woman or women


Severe Acute Respiratory Syndrome or SARS


electronic cigarettes or e-cigarettes

Terms that mean the same thing 

Cancer or neoplasms or tumour or tumor or carcinoma

Subject headings come from an established or controlled list of vocabulary and are assigned as ‘indexed terms’ to articles as they are added to medical databases. Subject headings bring uniformity and consistency to searches by using the same terminology for search terms, regardless of the language used by authors within individual studies.

Subject headings that you may be familiar with are MeSH (Medical Subject Headings) terms used in Medline and PubMed, and Emtree terms used in Embase. It is important to check the subject headings in the controlled vocabulary of each database you chose as not all databases use the same headings.

For example, if one of your main search concepts is ‘complementary medicine’

  • You would use the MeSH subject heading ‘complementary therapies’ in Medline
  • You would use the Emtree subject heading ‘alternative medicine' in Embase

Search for MeSH subject headings that relate to your keywords.

MeSH Database

Visit our overview of health databases to see a full list of databases and corresponding subject headings.

Health Databases Overview

3 minute guide to subject headings (2013), Leeds University Library

Some ways to identify synonyms of keywords and subject headings:

  • Check dictionaries and thesauri and encyclopaedias
  • Look up your concept in a general textbook or Clinical Decision Support tool to see how it is described and referred to. (Search the library catalogue or visit the Clinical Decision Support page)
  • Look for author keywords under the abstract
  • Check the citation records of relevant articles or from your ‘gold set’:
    • Ovid Medline has a ‘complete reference’ button. Click to view the indexed keywords and MeSH terms used in individual papers
    • Cochrane Library often lists the PICO search concepts and a list of MeSH terms used for individual papers
  • Search the MeSH Database to find relevant subject headings. Browse the subject tree and make a note of other relevant MeSH terms or ‘use for’ terms that can be added to your keywords.
  • Many systematic reviews are required to append a copy of their search strategies on publication. It can be helpful to look at the terminology that is used in reviews on related topics
  • Use text mining tools to find relevant keywords and subject headings based on your search concepts or your ‘gold set’
  • Don’t forget about alternate spellings, alternate endings and word forms of your keywords!

Quick links

Library Catalogue

Clinical Support Tools

MeSH Database

Text mining can help identify how often terms come up in the literature and help identify keywords and subject headings that are relevant to your search concepts.  Text mining tools are usually free and offer a quick way to find synonyms and fill gaps in your search. 

Text Mining tools

Locate synonyms by analysing word frequency or extracting keywords and subject headings used within an article.

  • Yale MeSH Analyzer - upload PMIDs to create a grid of MeSH terms and author keywords from individual papers.
  • MeSH on Demand - identifies MeSH terms in an uploaded text (abstract or full text).
  • PubMed Reminer -  provides analysis in a tabular format showing the frequency of terms and MeSH headings from search results.

Visualisation tools

Create word clouds and visual based graphics related to search terms. 

  • Carrot2 - enter keywords and get a visual overview of what's available grouped into topics. You will quickly find search terms and articles related to your search.
  • PubVenn - generates Venn diagrams based on keywords. It also links to articles within the PubMed databases.
  • Voyant Tools - web-based text analysis tool. Set of articles can be uploaded to the tool to show word frequency, word clouds etc. 

Visit the Text Mining page in the library’s Research Toolkit for more information and a list of helpful tools.

Text Mining Tools

Boolean operators are used to group and combine search terms. They are the most effective tool we have for searching databases because they turn complex search queries, with multiple concepts, into something the database can understand. 

Combining search terms

Two small words -- AND and OR -- are arguably the most common and powerful Boolean operators. 

OR - broadens your search by finding any results that contain either one of your terms e.g. cancer OR neoplasm.
AND - restricts your search by only finding results that contain both terms e.g. stroke AND aspirin.



Note: When used within a single search line, both AND and OR should be used to connect related themes or synonymous words, e.g. high blood pressure OR hypertension OR cardiovascular disease.

Grouping search terms

Parentheses (round brackets) are used to group together multiple search terms or concepts. Round brackets enclose distinct sections of the search, such as related terms combined with the OR operator. 

For example: (cancer OR neoplasm) AND (therapy OR treatment)

You are encouraged to document your search methodology. This helps structure your research and keep it on track. Record where you are searching, search terms being included (and excluded), your database search strategy and key references.

Recording search terms:

Create a concept map or table with a list of your main concepts (developed from your PICO), and keywords and subject headings that you have identified. It can be helpful to use a Word document table or an Excel spreadsheet.

You should start your list during the background research stage and add and adapt as you move through the literature searching process.

Note: Not all databases use the same subject headings, so you may need to include multiple lists of subject headings when searching across multiple databases.

Recording where you search

Document your search process to show and be reminded of how you located search results.  This can be outlined during the planning stage and is particularly helpful when working on research projects over a long period of time.

Use headings such as these:

  • databases and providers, e.g. Embase (Ovid)
  • date range of search 
  • filters or limitations used
  • search terms and subject headings
  • database-specific syntax and field codes used
  • AND/OR operators used to combine search terms
  • numbers of results

Save your database search

Save a copy of your search in the database that you are using. Having a search history ensures that you don’t have to repeat work and allows editing as your search progresses. You can create free accounts in many databases using your Monash Health employee login details. Visit the 'Working with Results' page for more information on saving your search in a database. 

Save your search results / references 

Export search results to a safe place to ensure that you don't misplace references, or forget where and how you found relevant references. Use reference management software such as EndNote. This helps you store, organize and retrieve search results quickly with options of attaching full text pdfs. Visit the 'Working with Results' page for more information on exporting results and managing references. 

Working with Search Results

Recording grey literature search results

Grey literature is an essential consideration in systematic and thorough searching. It encompasses government reports and policies, conference presentations and papers, issues papers and white papers, research and industry reports, theses and dissertations, trial data, statistics and surveys. 

Use this template to keep track of relevant results from your grey literature search. 

Visit the Grey Literature guide for more information on where top find grey literature and search tips. 

Grey Literature Guide

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