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Boolean Searching on the Internet

A Primer in Boolean Logic
Boolean Searching Using Search Engines
Quick Comparison Chart: Full Boolean vs. Implied Boolean vs. Templates
Where to Search: A Selected List

A Primer in Boolean Logic

The Internet is a vast computer database. As such, its contents must be searched according to the rules of computer database searching. Much database searching is based on the principles of Boolean logic. Boolean logic refers to the logical relationship among search terms, and is named for the British mathematician George Boole.

On Internet search engines, the options to construct logical relationships among search terms extend beyond the traditional practice of Boolean searching. This will be covered in the section below, Boolean Searching Using Search Engines.

Boolean logic consists of three logical operators:

  • OR
  • AND
  • NOT

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

OR

college OR university

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:

Search terms

Results

college

17,320,770

university

33,685,205

college OR university

33,702,660

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:

Search terms

Results

college

17,320,770

university

33,685,205

college OR university

33,702,660

college OR university OR campus

33,703,082


AND

poverty AND crime

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:

Search terms

Results

poverty

783,447

crime

2,962,165

poverty AND crime

1,677

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

For example:

Search terms

Results

poverty

783,447

crime

2,962,165

poverty AND crime

1,677

poverty AND crime AND gender

76

A few Internet search engines make use of the proximity operator NEAR. A proximity operator determines the closeness of terms within a source document. NEAR is a restrictive AND. The closeness of the search terms is determined by the particular search engine. For example, NEAR in AltaVista (Power Search) is 10 words. As another example, Google defaults to proximity searching by default.

NOT

cats NOT dogs

Query: I want to see information about cats, but I want to avoid seeing 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:

Search terms

Results

cats

3,651,252

dogs

4,556,515

cats NOT dogs

81,497

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.

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Boolean Searching Using Search Engines

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 first process this part of the search. Next, the search engine with combine this result with the last part of the search. Using this method, we are assured that the 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 well-known 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. Often 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

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Quick Comparison Chart: Full Boolean vs. Implied Boolean vs. Templates

Full Boolean

Implied Boolean

Template Terminology

OR

college or university

college university
*see note below

any of these words
can contain the words
should contain the words

AND

poverty and crime

+poverty +crime

all of these words
must contain the words

NOT

cats not dogs

cats -dogs

must not contain the words
should not contain the words

NEAR, etc.

cats near dogs

N/A

near

* 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.

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Where to Search: A Selected List

Feature

Search Engine

Boolean operators

HotBot | Ixquick | ProFusion ? Dogpile ? Google

Full Boolean logic with parentheses, e.g., behavior and (cats or felines)

HotBot | Ixquick | MSN Search Advanced Search

Implied Boolean +/-

Most search engines offer this option

Boolean logic by template terminology

Google Advanced Search | HotBot Advanced Search | Lycos Advanced Search | MSN Search Advanced Search ? Yahoo! Advanced Search

Proximity operators

Google [by default] | Ixquick

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Last Updated:  9/25/2015


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