Boolean Searching on the Internet

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-born Irish mathematician George Boole.

On Internet search engines, the options for constructing logical relationships among search terms extend beyond the traditional practice of Boolean searching. This will be covered in the section below, Boolean Searching on the Internet.

Boolean logic consists of three logical operators:

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.

Query:    I'm interested in dyslexia in adults.

Query:    I'm interested in radiation, but not nuclear radiation.

Query:    I want to learn about cat behavior.

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. Nowadays, most search engines default 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.

Query:    I'm interested in dyslexia in adults.

Query:    I'm interested in radiation, but not nuclear radiation.

Query:    I want to learn about cat 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

Query:    I'm interested in dyslexia in adults.

Query:    I'm interested in radiation, but not nuclear radiation.

Query:    I want to learn about cat behavior.

* Most multi-term search statements 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.