HELP: Boolean Searches
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Introduction to Boolean Searching
Boolean searching, based on a system of symbolic logic developed
by 19th century English mathematician George Boole, is a very
powerful way of searching a computer database. The
Media Catalog Search
and many Internet search programs support Boolean searches.
Boolean searching is not difficult, but it does require that you
think carefully about exactly what information you are seeking
from the computer and develop your search strategy accordingly.
There are some helpful hints you
should know before you start your Boolean search.
When you perform a Boolean search, you search the computer for
the keywords that best describe your topic. The unique power of
Boolean searching is that you combine these keywords using three
"operators".
Which operator you choose to combine
your keywords determines how the computer performs the search and
what information it returns to you.
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AND
This is a very specific or narrow search.
The operator AND tells the computer to search its database
for every entry (or record) that contains each of the
words we specify in the same record. For example, if we want to
find information on the 'supreme court', we might search the
appropriate computer in this way:
supreme AND court
The Venn diagram above illustrates what happens. The computer
goes through its database and retrieves every record it finds
with the word 'supreme' and every record with the word 'court'.
It then returns the list of records only in the shaded area where
the circles intersect. That is, those records in which
both words appear somewhere in the same record. You can
also perform an
advanced
AND search if you find
the database results are too broad.
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OR
This is a very broad search
and may retrieve a great many records.
When we combine our keywords using the operator OR we tell
the computer to retrieve every record that has at least one of
those words in it. Both do not have to appear. If either
word is present, the computer presents us with the record. For
example, if we wanted to find all the information the computer
has on either 'law' or 'legal', we might try this search:
law OR legal
As the diagram above shows, the computer again goes through its
database and finds every record with the word 'law' and every
record with the word 'legal'. It then presents us with every
record the database found which contains at least one (not
necessarily both) of the words we specified in our search.
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NOT
This is a selective search
which allows us to exclude unwanted records.
Combining our search terms with the operator NOT allows us
to strip a term out of our search, telling the computer we want
to see everything it has with the first word but nothing that
mentions the second word. For example, if we're looking for
material on the regional court system, we might want to exclude
records that pertain to the supreme court . We might try this
search:
court NOT supreme
Once more, the computer goes through
its database and extracts every record with the word 'court'. Of
that group of records the computer will display only those
records in which the word 'supreme' does NOT appear, as
the diagram above illustrates. We will not see any record that
has both words.
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Hints for Successful Boolean Searching
Before You Begin
Before you sit down at a computer to perform this type of search,
you should have thought carefully about what words you want to
search. We recommend that you develop a small vocabulary that
contains words describing your topic. This should include
synonyms for all your key concepts. If the computer finds nothing
using one word, try doing the same search again but substitute a
synonymous term. You may get quite different results.
Keywords
In addition to having your vocabulary prepared, you should
already have decided how you want to combine your keywords, i.e.
which operators you want to use with which words. Computers are
very literal-minded and very unforgiving. If you don't know what
you want to look for and how you want to look for it, the
computer will probably be of little use. You could easily spend a
great deal of time searching around and find nothing.
Using the operators
Using the operators AND and NOT will always narrow
your search, increasing your specificity and decreasing the
number of "hits", or records retrieved by the computer. The
operator OR broadens your search, increasing the number of
hits.
Using More Than One Operator
You may use more than one operator in a single search. For example:
This search would look for records that contain all three words:
judge AND court AND trial
In this search the computer would not include any record
with the first three words that also mentions 'supreme':
judge AND court AND trial NOT supreme
Order of Operations
Computers usually perform AND and NOT searches
first, and then OR searches. If we were looking for
information on courts in either USA or Canada, we might try this
search:
court AND USA OR Canada
Because the computer processes AND searches first, we
would end up with records that have the words 'court' and 'USA'
in the same record as well as all records containing the word
'Canada'. That would almost certainly not give us the result we
want.
Searchers may get around this by wrapping the OR words in
a parenthesis. This is called "nesting". It forces the
computer to search the parenthesized words first, before anything
else. So in order to run the above search correctly, we would
type in:
court AND (USA or Canada)
Truncation
Truncation allows us to search for a root word and all of its
various endings, such as plurals.
The symbol used for truncation is an asterisk (*).When you use
the
Media Catalog Search
, the asterisk (*) replaces several characters. For
example, if we want to find information on computer grapics, we
might try this search:
compu* AND graph*
Replacing the end of the word 'graphics' with an asterisk (*)
tells the computer to look for any endings to the root word
'graph' such as 'graphic' or 'graphical'. The same principal
holds true for replacing the last part of 'computer' with an
asterisk (*).
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Advanced 'AND'
Keyword searching is very powerful because it allows searchers to
combine two or more subjects, or specific aspects of subjects, to
find truly relevant items.
Rules to remember:
- If you get TOO MANY 'hits,'
add a term to make your search more
specific.
- If you get TOO FEW 'hits,'
take away a term to make your search less
restrictive.
Alternately, experiment with different words that
relate to your subject.
- If you're finding NOTHING, try
different search words.
This Venn diagram gives a visual conception of how two or more
subjects can intersect, creating an area of overlap that combines
multiple subjects.
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Examples:
Adolescents and Abuse and
Rehabilitation
Computers and Security and
Management
Pollution and Government and
Policy
Media and Politics and Canada
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Parts of this document were adapted, with permission, from
materials originally developed by St. Paul's School
(Concord, NH) in conjunction with McFarlin Library
(University of Oklahoma, Tulsa); and the University of
Pittsburgh, Pennsylvania whose help and generosity is
gratefully acknowledged.
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