Boolean logic consists of three logical operators:

• OR
• AND
• NOT

Each operator can be visually described by using Venn diagrams, as shown below.

Query: I would like information about college.

• In this search, we will retrieve records in which AT LEAST ONE of the search terms is present. We are searching on the terms college and also university since documents containing either of these words might be relevant.
• This is illustrated by:
  
  • the shaded circle with the word college representing all the records that contain the word "college"
  • the shaded circle with the word university representing all the records that contain the word "university"
  • the shaded overlap area representing all the records that contain both "college" and "university"

OR logic is most commonly used to search for synonymous terms or concepts.

Here is an example of how OR logic works:

OR logic collates the results to retrieve all the unique records containing one term, the other, or both.

The more terms or concepts we combine in a search with OR logic, the more records we will retrieve.

For example:

Query: I'm interested in the relationship between poverty and crime.

• In this search, we retrieve records in which BOTH of the search terms are present
• This is illustrated by the shaded area overlapping the two circles representing all the records that contain both the word "poverty" and the word "crime"
• Notice how we do not retrieve any records with only "poverty" or only "crime"

Here is an example of how AND logic works:

The more terms or concepts we combine in a search with AND logic, the fewer records we will retrieve.

For example:

A few Internet search engines make use of the proximity operator NEAR. A proximity operator determines the closeness of terms within the text of a source document. NEAR is a restrictive AND. The closeness of the search terms is determined by the particular search engine. Google defaults to proximity searching by default.

Query: I want information about cats, but I want to avoid anything about dogs.

• In this search, we retrieve records in which ONLY ONE of the terms is present
• This is illustrated by the shaded area with the word cats representing all the records containing the word "cats"
• No records are retrieved in which the word "dogs" appears, even if the word "cats" appears there too

Here is an example of how NOT logic works:

NOT logic excludes records from your search results. Be careful when you use NOT: the term you do want may be present in an important way in documents that also contain the word you wish to avoid.

BOOLEAN SEARCHING ON THE INTERNET

When you use an Internet search engine, the use of Boolean logic may be manifested in three distinct ways:

1. Full Boolean logic with the use of the logical operators
2. Implied Boolean logic with keyword searching
3. Predetermined language in a user fill-in template
   

1. Full Boolean logic with the use of the logical operators
Many search engines offer the option to do full Boolean searching requiring the use of the Boolean logical operators.

Examples:

Query: I need information about cats.
Boolean logic: OR
Search: cats OR felines

Query: I'm interested in dyslexia in adults.
Boolean logic: AND
Search: dyslexia AND adults

Query: I'm interested in radiation, but not nuclear radiation.
Boolean logic: NOT
Search: radiation NOT nuclear

Query: I want to learn about cat behavior.
Boolean logic: OR, AND
Search: (cats OR felines) AND behavior

Note: Use of parentheses in this search is known as forcing the order of processing. In this case, we surround the OR words with parentheses so that the search engine will process the two related terms first. Next, the search engine with combine this result with the last part of the search that involves the second concept. Using this method, we are assured that the semantically-related OR terms are kept together as a logical unit.

2. Implied Boolean logic with keyword searching
Keyword searching refers to a search type in which you enter terms representing the concepts you wish to retrieve. Boolean operators are not used.

Implied Boolean logic refers to a search in which symbols are used to represent Boolean logical operators. In this type of search on the Internet, the absence of a symbol is also significant, as the space between keywords defaults to either OR logic or AND logic. Many popular search engines traditionally defaulted to OR logic, but as a rule are moving away from the practice and defaulting to AND.

Implied Boolean logic has become so common in Web searching that it may be considered a de facto standard.

Examples:

Query: I need information about cats.
Boolean logic: OR
Search: cats felines

This example holds true for the search engines that interpret the space between keywords as the Boolean OR. To find out which logic the engine is using as the default, consult the help files at the site. Nowadays, there are few engines that use OR logic as the default.

Query: I'm interested in dyslexia in adults.
Boolean logic: AND
Search: +dyslexia +adults

Query: I'm interested in radiation, but not nuclear radiation.
Boolean logic: NOT
Search: radiation -nuclear

Query: I want to learn about cat behavior.
Boolean logic: OR, AND
Search: cats felines +behavior

3. Predetermined language in a user fill-in template
Some search engines offer a search template which allows the user to choose the Boolean operator from a menu. Usually the logical operator is expressed with substitute language rather than with the operator itself.

Examples:

Query: I need information about cats
Boolean logic: OR
Search: Any of these words/Can contain the words/Should contain the words

Query: I'm interested in dyslexia in adults.
Boolean logic: AND
Search: All of these words/Must contain the words

Query: I'm interested in radiation, but not nuclear radiation.
Boolean logic: NOT
Search: Must not contain the words/Should not contain the words

Query: I want to learn about cat behavior.
Boolean logic: OR, AND
Search: Combine options as above if the template allows multiple search statements

* This search statement will resolve to AND logic at search engines that use AND as the default. Nowadays most search engines default to AND. Always play it safe, however, and consult the Help files at each site to find out which logic is the default.