Using Keywords and Boolean Logic or Connectors
Scope: Do you ever have trouble finding information that seems relevant to your search in the Libraries' Catalog or a database? You can use Boolean Operators to narrow or broaden your searches. This guide gives a brief overview of how to use Boolean Operators. Contact a librarian in libues[at]umd.edu for more information.
Table of Contents
What is Boolean Logic?
Boolean logic is a system that allows a searcher to communicate to a database specific relationships between keywords (or concepts) when searching. The most commonly used Boolean commands (sometimes called logical operators) are AND, OR, and NOT.
Where is it Used?
It is used in databases:- Web: Boolean logic is used in search engines to retrieve items, although you may not even be aware that it is being used-i.e. often it is implied or entered automatically. For example, when searching in Google, if you enter words adjacent to each other, say medical marijuana, the interface automatically inserts an AND operator between the words and returns documents or items which contain both of these words, not necessarily adjacent to each other. Because different search engines have different default commands (some default to OR), it is important to understand what they are. This can generally be discovered by consulting the help or search tips menu found in most databases.
- Library Catalog: Like Google, the University catalog uses implied Boolean commands and inserts AND between terms, resulting in the retrieval of items which contain both of your keywords.
- Periodical Databases: Periodical databases (access these through Research Port) use Boolean language as well. These, however, do not generally insert a default command between your keywords. Instead, if you enter two words adjacent to each other in a search box, they will interpret them as a phrase and look for exactly those words in that order. So, if you put in medical marijuana, the database will search through its records and look for that exact phrase. If it does not find that phrase in any documents, no results will be returned.
It's important to know how Boolean commands operate so that you can understand what you are asking for and how to craft an effective search strategy.
How do I Use It?
To illustrate Boolean searching, let's assume that you have decided to write a research paper on the controversy surrounding the medical use of marijuana. The research question might be formulated as:
1. Begin by identifying the main keywords or concepts in your topic: medical and marijuana.
2. Now that you have identified the keywords, you need to communicate to the database what you are looking for. This is done by using the Boolean commands AND, OR, and NOT.
The Use of AND
AND is used to tell the database that you want to retrieve documents or items which contain both (or all) of the keywords you specify. It is used to connect unrelated terms or concepts. A search (using the advanced search interface in Academic Search Premier) using AND with these keywords would look like this:
And the results would be:
AND has the effect of narrowing or restricting the number of documents retrieved. If we were to add in another term, such as controversy, the number of documents retrieved would shrink down to only those which contain all three terms.
The Use of OR
OR is used to communicate to the database that you want to retrieve documents which contain one, or the other, or all of the terms that you specify. It is used to connect related or synonymous terms and is a very powerful tool in keyword searching. Its effectiveness lies in the fact that several different terms can be used to express a concept. For example, the concept of 'children' can also be expressed using the terms 'kids' or 'youth'. In keyword searching, if you don't specifically ask for the synonymous terms, you won't get results which contain them. In relation to our search question, some authors may use the term 'medical' while others may use 'therapeutic'; some authors may use 'marijuana' while others use 'cannabis'. (It is at this stage in developing a search strategy that a thesaurus can be very helpful.) OR allows you to ask the database to retrieve all these variants. For example, an OR search in CQ Researcher would look like this:
And the results would be:
Note: When you combine AND and OR in the same search, use parentheses around the OR phrase.
OR has the effect of expanding or increasing the number of documents retrieved. The more synonymous terms you add to your search, the greater the number of results you will retrieve.
The Use of NOT
NOT is used to communicate to the database that you do not want the items it retrieves to contain certain terms that you specify; that is, it excludes results. For example, you are searching in a periodical database for newspaper articles about your topic but you don’t want to retrieve any editorials. Your search would look like this in the Alternative Press Index:
And the results would be the following:
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