Identifying search terms (keywords and subject headings)


Keywords are words that come to you naturally, or that may be part of a specific discipline vocabulary (e.g. terms used only by midwives), or that you brainstorm when planning your search. If the 'map term to subject heading' box is ticked the database will attempt to map your keyword or phrase to a subject heading in the database’s thesaurus. If the ‘map term to subject heading’ box is not ticked, the word or phrase will be treated as a keyword. Searching by keyword finds only those results where your keyword appears as an exact match in several fields including the title or abstract. This works particularly well if you are looking for a specific spelling, product, term, or phrase.

Remember, it is important to recognise that different spelling and terminology may exist for the same search topics. Keyword searching will not differentiate between spellings.

An example of keyword searching:

Consider the concept ‘bushfire’.

This word has a particular application to the Australian context, where it is used to describe what other countries may call ‘forest fire’ or ‘wildfire’. Because of its very specific context, ‘bushfire’ is unlikely to have an associated subject heading in databases such as MEDLINE and Embase.

By unselecting (un-ticking) the Map to Subject Headings option in these databases, the search will look for the word ‘bushfire’ appearing in titles and abstracts of the literature. You will see that the results are primarily Australian based studies or references.

Is there a word or term that is very specific to your area of practice? Try searching for it as a Keyword search and view the results.

Subject headings

Subject headings are terms that have been identified and defined to cover a particular concept. Synonyms (alternate words that may also be used for that concept) are then ‘mapped’ to those subject headings.

Databases like MEDLINE and Embase use a thesaurus to group related concepts together. In MEDLINE, the thesaurus is known as Medical Subject Headings (MeSH); in Embase the thesaurus is known as Emtree.

An example of subject heading search:

Consider the concept: ‘heart attack’.

Synonyms for ‘heart attack’ include myocardial infarction, heart infarction, subendocardial infarction, anterior myocardial infarction, inferior myocardial infarction; even ischaemic heart disease, angina, ST segment elevation, cardiogenic shock, and many more – all of which may be embraced within the concept ‘heart attack’.

In MEDLINE, the MeSH term for ‘heart attack’ is Myocardial Infarction, but because of the mapping that has been done to associate the synonyms with MeSH terms, any search for ‘heart attack’ or any of its synonyms would automatically bring up (or be ‘mapped to’) the term ‘Myocardial Infarction’.

Subject headings are assigned to every concept within an article by people working as Indexers, using the database thesaurus. A thesaurus may also be referred to as a controlled vocabulary or index term.

Using the MeSH or Emtree subject headings is both efficient and effective because they search for matching content across a range of synonyms (rather than just single words as in Basic Search).

Comparing Subject Headings with Keywords

Subject Headings Keywords
Pre-defined ‘controlled vocabulary’ words assigned to describe the content of each item in a database or catalogue Natural language words describing your topic
Database looks for subjects only in the subject heading or descriptor field, where the most relevant words appear Database looks for keywords anywhere in the record (title, author name, subject headings, etc.)
If a subject heading search yields too many results, you can often select subheadings to focus on one aspect of the broader subject Often yields too many or too few results
Results are usually very relevant to the topic May yield many irrelevant results
Will find synonyms, plurals, spelling variations Will not pick up synonyms, plurals, or spelling variations
Subject headings will locate most of the relevant papers, but for a comprehensive search you may need to supplement your subject heading searches with some keyword searches Necessary when there is no subject heading available for the concept you wish to search

Note that a search may include both subject heading and keyword approaches.