Keyword searches return documents that include the word you searched for. Let's imagine a keyword search in a database:
The 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:
Scholars who need to do a thorough literature search on a topic are greatly assisted by a special kind of tool called a controlled vocabulary. 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.