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Systematic Reviews & Other Syntheses


Some databases have the option to search using subject headings, also referred to as subject terms, controlled vocabulary, and indexed terms. Subject headings are a set of pre-determined terms (a controlled vocabulary) describing concepts within a specific database. In most cases, subject headings are applied to database records by human indexers who have read the full-text to understand the main subjects discussed, although some databases use automation as well.

The following page works through subject heading search options available on the Ovid platform that is used for a number of databases such as MEDLINE, Embase, EBM Reviews (i.e. Cochrane Library), PsycINFO, etc. The examples and screenshots provided are from Ovid MEDLINE and may function or appear slightly different on other Ovid databases. Similar principles apply to other search platforms.

Why use subject headings to search?

  • Subject headings identify the main topics discussed in indexed records, which can retrieve more relevant results than searching using keywords

(Note: searching without subject headings is referred to as keyword searching. By default, many databases search for your keyword search terms in the bibliographic details of the record i.e. the article title, abstract, author keywords, as well as in the database-specific subject headings)

  • Subject headings can help eliminate some of the guess work for the searcher by using a controlled vocabulary to search for concepts rather than specific words that appear in the bibliographic details. This helps to capture records with alternate spellings and records that use different terms to describe the same concept

For example: the terms physical activity and exercise can be used to describe the same concept. The database will have a pre-determined term (subject heading) to search for records that are about the concept of physical activity or exercise, regardless of what words are used to describe the concept in the bibliographic details. For the same reason, we don’t have to worry about different spellings (ex. paediatrics vs pediatrics) for the same concept when searching with subject headings

  • Most subject headings are organized into a hierarchy that helps you identify and easily include more specific subject headings

For example: searching the subject heading that the MEDLINE database uses for the concept of exercise, presents a list of more specific types of exercise that you can easily choose to include or leave out (see further down the page)

Mapping to Subject Headings

  • To search with subject headings when available in Ovid databases, make sure the box is checked for Map Term to Subject Heading underneath the search box
  • Enter one of your search concepts and search (you will combine search concepts later):


  • Ovid will map you to a list of the best-matching subject headings associated with your term
  • Note that scope notes are available for a description of all subheadings
  • Do not select continue from this page. Instead, click on the hyperlink for the best subject heading for your concept


  • Selecting the hyperlinked subject heading will allow you to view the hierarchy of the databases controlled vocabulary and select more specific subject headings to include, if desired

  • Note: if Ovid does not map you to an appropriate subject heading, go back to the advanced search screen and try using a different term for the same concept
  • Note: the database may not have an appropriate subject heading for some less commonly described concepts in the literature. In this case, you can uncheck the Map Term to Subject Heading box and search for the concept as a keyword in the bibliographic details


  • In MEDLINE and other databases that utilize subject headings, subheadings may be assigned to subject headings to further narrow the scope of the concept
  • When conducting a synthesis search, it is advisable to use a broad search by including all subheadings (the default if you continue without checking any subheadings) 

  • Note that scope notes are available for a description of all subheadings

Explode and Focus

  • Explode and Focus are best utilized from the page that shows the hierarchy of terms and the number of citations assigned to a term in order to make informed decisions about the terms included and not included
  • Explode a subject heading to retrieve results with that subject heading as well as all of its narrower, more specific headings (broadens the search)
    Note: not all subject headings have narrower, more specific headings (even though the search box for the explode option always appears, as it does here for Pediatric Obesity, even though there are no narrower terms indented below)
  • Focus is not advisable for comprehensive searching. Focus limits a subject heading to retrieve results in which this term is one of the major main topics of the article (narrows the search)