Keyword searches return documents that include the word you searched for. Let's imagine you searched for the word "Phthises" in a database:
Your search results could include:
Because keyword searches look for words, not ideas, you can miss important documents when authors use alternate spellings and synonyms. Unless you can think of all the possible alternate words that describe your topic, your search will be incomplete. In order to construct a thorough literature search with keywords, you would have to search with all of these different spellings and synonyms:
Controlled vocabularies group synonymous words together under one main term: a Subject Heading. Searching with Subject Headings means researchers find articles that are about their topic, rather than articles that use specific words. In addition, many controlled vocabularies are arranged in a hierarchy or tree that shows the relationships between Subject Headings. This allows searchers to easily understand how their topic relates to others.
If we go back to the example from above, the MeSH term for older people is "Aged." If you wanted to use a combination of keywords and subject headings, you could search:
Keywords are great for newer topics where a subject heading hasn't yet been created, or if you are just doing a quick search. Subject Headings are best for doing deeper searches for established content, getting more precise results (including using sub-headings), and controlling for alternate spellings and acronyms.
The most comprehensive way to build a search is to use a combination of keywords and subject headings. You can combine your synonyms using the Boolean Operator OR. Learn more about building boolean searches in the "Building Your Search" section.
You can use the database’s thesaurus to find subject terms. Different databases may use different subject headings, so it is a good idea to check what terms the database actually uses. MEDLINE (PubMed and Ebsco) uses Medical Subject Headings (MeSH), while CINAHL uses their own specialized CINAHL Subject Headings.