In many cases, keyword searching will be suitable for conducting a systematic review. However, it might also be necessary to conduct a subject heading search too. Subject heading searches are different to keyword searches. A keyword search will search the entire record of a document looking for any term or a closely related word, regardless of whether or not it fully describes the document's contents.
In contrast, when a document, such as an article, is added to a database or index, the cataloguer will assign or tag specific terms to that document from a list of approved, standardised terms. These terms are often called subject headings - although, sometimes they can be refer to as MeSH headings, thesaurus terms or controlled vocabularies. These subject headings are clearly defined and will only be assigned to the document if they are relevant to the contents of the document in question.
Subject headings are often separated into major and minor categories. Subject headings can often be searched within specific databases, so that it is possible to see all the documents associated with particular subject headings.
Example of subject headings for a journal article:
|Article title||Major subjects||Minor subjects|
|What makes a mobile app successful in supporting health behaviour change?||
Privacy and confidentiality
When conducting a keyword search the term that you use may appear in the search results, but may not be the primary focus of the research - it just happens to mention the term. The resultant search results might fit your search, but might not be relevant to your topic.
Subject headings, on the other hand, are useful as you can search for the overarching subject heading but also all the terms related to the main heading, meaning you capture all the alternative keywords linked to that concept. For example, a subject heading search for myocardial ischemia might also include other coronary diseases, such as angina pectoris, coronary arteriosclerosis, coronary vasospasm and so on. This will ensure that relevant keywords to your search are not overlooked.
A search can use a combination of keyword and subject heading searches.
The table highlights the strengths and weakness of using subject searching.
|Advantages of subject heading searching||Disadvantages of subject heading searching|
|It will find material on the same topic even when the material uses different terms and keywords.||Different databases use different subject heading systems, so headings will vary between databases.|
|It will focus your search and help to reduce the amount of irrelevant information.||Subject headings might not be immediately assigned to new articles - sometimes there is a time lag in the article being fully tagged.|
|It will reduce the likelihood of missing relevant information.||It is not as flexible as keyword searching, as it relies on having some prior knowledge of the topic and associated terminology.|
If you intend to search using subject headings, you will need start by finding the relevant subject headings related to your topic, as different databases may use different sets of subject headings.
To find prescribed subjects headings, conduct a keyword search using the appropriate subject headings search box, and then browse the list of subject headings to locate the most relevant ones for your topic.
Alternatively, you can run a simple keyword search on your topic first, then look at the results to find a few relevant articles. Each entry should display which subject headings have been assigned to these articles which you can then use to build your subject heading search.
Some databases have search builder that allows you to add subject headings to your search. You should also be able to add additional keywords at some point in the process to create the most robust search for your area of research. Alternatively, you may be able to add the headings manually to the search boxes and select the appropriate search field from the drop down menu.
Databases will often have help pages if you're not sure how they function with subject searching. CINAHL, for example, has instructions for subject searching. These instructions will also work with MEDLINE (Ebsco).
Subject headings may be identified minor or major headings. If the subject heading describes the main focus of the information, then it is a major heading of that article. If the article includes information about the subject but it is not the main focus, the subject heading may be assigned to it as a minor heading. You can choose to search for a subject heading as a major heading.
You can often 'explode' a subject heading so it will search for the overarching concept and all the other narrower terms related to the heading that come underneath it. This makes it easy to look for all related terms within a topic which may be missed in keyword searching and will increase your results.
E-learning for healthcare and NHS Health Education England. (no date) How to search the literature effectively: a step-by-step guide for success: module 6: searching with subject headings. [Online] [Accessed on 24th April 2019] https://portal.e-lfh.org.uk/LearningContent/LaunchForGuestAccess/512698
Ebsco Connect. (2018) What are major subject headings and minor subject headings? [Online] [Accessed on 8th April 2020] https://connect.ebsco.com/s/article/What-are-Major-Subject-Headings-and-Minor-Subject-Headings?language=en_US
Ebsco Connect. (2019) Using CINAHL/MeSH subject headings [Online] [Accessed on 8th April 2020] https://connect.ebsco.com/s/article/Using-CINAHL-MeSH-Headings?language=en_US
Massachusetts Institute of Technology Libraries. (no date) Database Search Tips: Keywords vs. subjects. [Online] [Accessed on 24th April 2019] https://libguides.mit.edu/c.php?g=175963&p=1160804
Nova Southeastern University. (2018) Database search tips: subject searching. [Online] [Accessed on 24th April 2019] https://nsufl.libguides.com/c.php?g=112492&p=725946