Generate and Combine Search Terms

Controlled Vocabulary

Controlled vocabularies are a set of predetermined terms that describe specific concepts. You will find that many databases use their own controlled vocabularies (sometimes called index terms, subject headings, or a thesaurus) to enhance the findability of citations. If you have heard of MeSH, this stands for Medical Subject Headings, and is the controlled vocabulary used in PubMed.

Databases that use controlled vocabularies employ subject specialists who review individual citations and add the appropriate controlled vocabulary terms to the citation that describe all of the concepts covered in the article. Using controlled vocabulary terms in your search strategy allows you to locate citations no matter what term(s) an author does or does not use, and helps account for spelling variations and acronyms.

Methods for Finding Controlled Vocabulary Terms

  • Do a basic keyword search and choose a few citations that are relevant to your research question.
  • Make a note of any keywords used in the article title or citation you may wish to add to your current list of keywords.
  • Examine the full citation information for each article to find the controlled vocabulary terms assigned and write them down.
  • Redo your search using both the controlled vocabulary and keyword terms.


Keywords are the words used in an article title, abstract, or other text field in a database. Keyword searching, or natural language searching, is how most people search for information and is often sufficient for most people. One drawback of searching with keywords is that the words that you use must match the terms used by an author. To remedy this problem, a complete keyword search strategy will include multiple spellings and synonyms that represent the concept. Keyword searching is also useful when attempting to identify literature that may not have been indexed with controlled vocabulary terms, for any variety of reasons.

Combining Search Terms Effectively

See the Search Filters Guide for examples of PubMed and other database searches that combine controlled vocabulary and keyword terms effectively.

Boolean operators form the basis of mathematical sets and database logic. Using these terms to combine concepts and keywords in your search will assist you in performing a transparent, reproducible, and thorough search. Boolean operators include AND, OR, and NOT. These are generally always capitalized.

  • AND - AND is used to link ideas and concepts where you want to see both ideas or concepts in your search results. AND narrows the search.

“children” AND “poverty”

  • OR - OR is used between similar keywords, like synonyms, acronyms, and variations in spelling within the same idea or concept. OR broadens the search.

“behavior” OR “behaviour”

  • NOT - NOT is used to exclude specific keywords from the search, however, you will want to use NOT with caution because you may end up missing something important. NOT is used to make broad exclusions. In this example, any article that discusses both “pediatrics” as well as “adults” will be excluded.

“pediatrics” NOT “adults”

Searching Best Practice: Combine Controlled Vocabulary with Keywords

It is good practice to search with both controlled vocabulary and keywords. Here are a few reasons:

  • Not every database uses a controlled vocabulary (Scopus and Web of Science are two examples)
  • Each database may have its own controlled vocabulary terms
  • If an article is too new, it may not be indexed with controlled vocabulary yet, and you would only find it with keywords
  • In PubMed, if an article is out of scope of MEDLINE, a component of PubMed, it may not ever be indexed with MeSH (PubMed’s controlled vocabulary), and you would only find it with keywords
  • If an article lacks an abstract or additional author-supplied keywords, you may only find it with controlled vocabulary

If you want to be as comprehensive as possible, search with both controlled vocabulary and keywords.

Use Caution

In bibliographic databases, stop words are words that the database has been programmed to ignore in a search string or query. Stop words include of, the, is, at, which, and on. Avoid using stopwords in your searches.