Literature Reviews

Boolean operators are the core of a successful and targeted literature search. Boolean operators are three simple operators you can use to connect your search terms and instruct the database on how to look for literature in a way that fits your research question.

AND

• use AND in between search terms to tell the database that your results must include both terms. This is a great way to narrow your results down if you are retrieving too many non-relevant articles.
• Use AND to connect the concepts in your question together.
• Ex: If you want to search for articles on diabetes in children, you would search for diabetes AND children

OR

• Use OR in between search terms to tell the database that your results can include either or both of the search terms.
• Use OR to connect synonyms and related concepts to get a more thorough search
• Ex. To get a full search for all child populations you would search for children OR youth OR minors 

NOT

• Use NOT to tell the database that you do not want articles including a search term within your results.
• This can be used to exclude things irrelevant to your research, such as certain populations.
• Ex. If we wanted to search for children but not teens you could search for children NOT teens

There are three more important search techniques vital to developing a good search string. These techniques are used to structure your search and get exactly the results you need.

• Parentheses
  • Use parentheses to group terms and tell the database what combination you want to search for and in what order. This is similar to algebraic expressions.
  • This is how you combine different types of boolean operators and structure you search in line with your research question
  • For example (children OR youth) AND (diabetes OR obesity) would first search the elements in parentheses, and then combine the results of those two searches with an AND
• Quotation Marks
  • Use quotation marks around a phrase to tell the database to search for those exact words in that exact order. This is required whenever you are searching for a phrase instead of just a single keywords.
  • Ex. "mental health"
• Wildcards/Truncation
  • Use * to tell the database to search all terms that start with a particular string of letters. For example, pharm* would search for pharmacology, pharmaceuticals, pharmacologists, and more.
  • This is a great way to expand your search without doing more typing

((healthcare OR "health care") AND ethics) NOT nurses

• As an example, the above search string would search for articles including either healthcare or health care and the word ethics, and exclude every article mentioning nurses.

Subject headings, also known as thesauri, controlled vocabularies, or taxonomies, are a set of specific and controlled terms applied to research articles to clearly mark what topics a research article addresses.  Subject headings:

• Are used to help researchers find all relevant articles on a particular topic
• Search for all articles on a topic regardless of variations in terminology
• Are a predefined set of words developed by database vendors and professional indexers

As an example, let's look at MeSH, the subject heading system used by PubMed. The MeSH term Drug Therapy is used for any articles that reference drug therapy, drug therapies, chemotherapy, chemotherapies, pharmacotherapy, and pharmacotherapies. So when you are using PubMed, you can do a search for the subject heading Drug Therapy to get comprehensive results, even if the author was using the alternative terminology listed above.

Subject headings are a great way to get a comprehensive search without thinking of every possible synonym or language variation on your own, which also makes them ideal for quick searches on specific topics.

To use subject headings:

• Find a way to browse a particular database's subject headings, such as the MeSH browser for PubMed 
• Do keyword searches to look for relevant subject headings
• Click on subject headings of interest and read their scope notes/documentation to make sure it is what you need
• Add them to your search with boolean operators and with the correct field tags to identify them as subject headings

Note that every database uses its own subject headings system. When using a new database, try looking for terms such as Subject Headings, Index Terms, or Thesaurus in its navigation or help menus to find their subject headings. Database help or support pages may also provide guidance on using subject headings.

Turning a research question into a search string is a multi-stepped process:

1. Break down your research question into searchable concepts
2. Write down a list of keywords and synonyms for each concept
3. Map each concept to relevant controlled vocabularies for each database (such as MeSH)

As you do test searches and read more relevant literature, you will be able to add additional keywords to your search concepts. It is recommended that you reach out to a librarian for assistance in generating keywords and mapping concepts to a controlled vocabulary.

Example Research question: Does obesity lead to heart disease and strokes?

Once all terms have been identified, you need to put them together in a search string. You can export your search strategy in addition to the results, to use in your search documentation.

A search string will generally look like:

(Topic A term 1 OR Topic A term 2) AND (Topic B term 1 OR Topic B term 2) AND (Topic C term 1 OR Topic C term 2)

If searching PubMed with our example research question, the search string would look like:

(obesity OR overweight OR obese OR "morbidly obese" OR "Obesity"[Mesh] OR "Obesity, Morbid"[Mesh]) AND ("heart disease" OR "cardiac diseases" OR "heart disorders" OR cardiovascular OR "Heart Diseases"[Mesh]) AND (stroke OR "cerebrovascular accidents" OR "Stroke"[Mesh])

The search string above was developed for PubMed. When adapting the string for another database, you want to have the strings operate as similarly as possible. You would replace the MeSH terms with the controlled vocabulary of the other databases used.

The search string above is searching with both keywords and MeSH terms. The MeSH terms will be searched in the MeSH field. The keywords will be searched in all fields, like the title, abstract, journal name, etc.

In a systematic review, you will need to keep the search as similar as possible between different databases. In practice, you should only need to change a few things for each database:

• Applying consistent search limiters to each database
• Searching the same fields in each database
• Applying the controlled vocabulary of each database

As experts in database searching, librarians can help you translate your search string between different databases.