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Skills Help Menu - Search
the Web
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Boolean Logic
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| 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. |
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| Boolean logic consists of three logical operators: |
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Each operator can be visually described by using
Venn diagrams, as shown below.
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OR
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college OR
university
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| Query: I would like information
about college. |
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- 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
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| OR logic is most commonly used to search for synonymous
terms or concepts. |
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| Here is an example of how OR logic works: |
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| Search terms |
Results |
| college |
17,320,770 |
| university |
33,685,205 |
| college OR university |
33,702,660 |
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| OR logic collates the results to retrieve all
the unique records containing one term, the other,
or both. |
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| The more terms or concepts we combine in
a search with OR logic, the more records
we will retrieve. |
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| 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 |
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AND
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poverty AND
crime
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| Query: I'm interested in
the relationship between poverty and crime.
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- 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"
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| Here is an example of how AND logic works: |
| Search terms |
Results |
| poverty |
783,447 |
| crime |
2,962,165 |
| poverty AND crime |
1,677 |
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The more terms or concepts we combine
in a search with AND logic, the fewer records
we will retrieve.
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| For example: |
| Search terms |
Results |
| poverty |
783,447 |
| crime |
2,962,165 |
| poverty AND crime |
1,677 |
| poverty AND crime AND gender |
76 |
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| 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. NOT is a restrictive AND.
The closeness of the search terms is determined
by the particular search engine. Google defaults
to proximity searching by default. |
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NOT
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cats NOT
dogs
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| Query: I want information
about cats, but I want to avoid anything about dogs. |
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- 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
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| Here is an example of how NOT logic works: |
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| Search terms |
Results |
| cats |
3,651,252 |
| dogs |
4,556,515 |
| cats NOT dogs |
81,497 |
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| 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 on the Internet
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| When you use an Internet search engine, the use
of Boolean logic may be manifested in three distinct
ways: |
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- Full Boolean logic with the use of the logical
operators
- Implied Boolean logic with keyword searching
- Predetermined language in a user fill-in template
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| 1. Full Boolean logic with the use of
the logical operators |
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| Many search engines offer the option to do full
Boolean searching requiring the use of the Boolean
logical operators. |
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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.
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| 2. Implied Boolean logic with keyword searching |
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| Keyword searching refers to a search type
in which you enter terms representing the concepts
you wish to retrieve. Boolean operators are not
used. |
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| 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. |
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| Implied Boolean logic has become so common in
Web searching that it may be considered a de facto
standard. |
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| Examples: |
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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
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Quick Comparison Chart:
Full Boolean vs. Implied Boolean vs. Templates
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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 |
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* 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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