Boolean logic is a system of showing relationships between sets by using the operators AND, OR, and NOT.
The term Boolean comes from the name of the man who invented this system, George Boole.
Boolean operators will help you broaden and narrow your searches when when searching library catalogs, databases, and the web.
A good way to illustrate how boolean logic works is through a Venn diagram. The circles in a Venn diagram illustrate different sets and the shaded areas show how the boolean terms form relationships between the sets.
Below are examples of Boolean operators and their associated Venn diagrams.
mushing AND racing
The blue area represents search results that contain information about mushing AND racing.
The white part of the circles represent items that have information about mushing but not racing, or racing but not mushing.
Only the items in the blue area will be retrieved.
The Boolean operator AND narrows a search.
Use AND when you are searching for different main concepts.
caribou OR reindeer
The blue area represents search results that contain information about caribou OR reindeer.
The Boolean operator OR broadens a search.
Use OR to search for synonyms/related terms.
fisheries NOT Alaska
The blue area represents search results that contain information about fisheries, but NOT Alaska.
The Boolean operator NOT eliminates all results containing a specific word, so be very careful using the NOT Boolean operator.
Finding the right number of search results can be tricky.
Sometimes search tools bring up thousands of hits, when all that is needed is one. Sometimes only a couple of results emerge from a search when you need as many as possible. Sometimes you want just enough sources for a good two, ten or 20 page paper-- but how many is that?
It depends on the context and on the quality or relevance of the search results. Chances are you will only use a fraction of the results that emerge from a single search.
Boolean operators can help you manage the number of search results retrieved.
Searching for the term mushing by itself may bring up relevant hits, but it may retrieve too many hits.
Narrow your search by adding another concept.
For example, if you're looking for information about racing and mushing (as opposed to recreational mushing), try the Boolean search:
This search forces the search engine to find information containing both terms, not just one term.
Let's take a look at this in Google. Search for the term mushing, which brings up approximately 12,800,000 results.
Narrow the search by adding the concept of racing using the AND Boolean operator.
Unlike library databases, Google automatically ANDs terms.
In Google you don't need to type the word AND, but we did for demonstrative purposes.
The second search results in fewer hits because AND narrows a search.
Searching for the term caribou by itself may bring up relevant hits, but it may not retrieve articles that use the term reindeer instead of caribou.
Broaden your search by using OR to search for synonyms and related terms.
For example, if you're looking for information about caribou, be sure to include the synonymous term reindeer.
This search tells the search engine to find information containing either term.
Let's take a look at this in Google. Search for the term caribou, which brings up approximately 62,700,000 results.
Broaden the search by using OR to search for reindeer, a synonymous term for caribou.
The second search results in more hits. This is because OR broadens a search.
Sometimes you'll need to create a complex search that uses AND as well as OR.
When writing complex Boolean search strings in Google, OR terms must be grouped in parentheses.
The NOT Boolean operator is used to eliminate items containing specific words.
For example, let's say you're searching for information about immigration reform in America. Your search string looks like:
"immigration reform" AND America
You keep getting results having to do with Latin America.
In this case you can cautiously include the NOT Boolean operator in your search.
When using library databases, use the term NOT. When using Google, use a minus sign.
This is what the search would look like in Google:
Different library catalogs and databases handle boolean expressions differently!
Read database help information to clarify how to best write your search statement.
Taking a few moments to read through the "how to search" instructions may save you hours of time and frustration.