Linking terms (also called Boolean operators) form the basis of mathematical sets and database logic.
They connect your search words together to either narrow or broaden your set of results.
The three basic boolean operators are: AND, OR, and NOT.
Why use linking terms?
To focus a search, particularly when your topic contains multiple search terms.
To connect various pieces of information to find exactly what you're looking for
Use AND in a search to:
narrow your results
tell the database that ALL search terms must be present in the resulting records
example: cloning AND humans AND ethics
The purple triangle in the middle of the Venn diagram below represents the result set for this search. It is a small set using AND, the combination of all three search words.
Be aware: In many, but not all, databases, the AND is implied.
For example, Google automatically puts an AND in between your search terms.
Though all your search terms are included in the results, they may not be connected together in the way you want.
For example, this search: college students test anxiety is translated to: college AND students AND test AND anxiety. The words may appear individually throughout the resulting records.
You can search using phrases to make your results more specific.
For example: "college students" AND "test anxiety". This way, the phrases show up in the results as you expect them to be.
Use OR in a search to:
connect two or more similar concepts (synonyms)
broaden your results, telling the database that ANY of your search terms can be present in the resulting records
example: cloning OR genetics OR reproduction
All three circles represent the result set for this search. It is a big set because any of those words are valid using the OR operator.
Use NOT in a search to:
exclude words from your search
narrow your search, telling the database to ignore concepts that may be implied by your search terms
example: cloning NOT sheep
Databases follow commands you type in and return results based on those commands. Be aware of the logical order in which words are connected when using Boolean operators:
Databases usually recognize AND as the primary operator, and will connect concepts with AND together first.
If you use a combination of AND and OR operators in a search, enclose the words to be "ORed" together in parentheses.
ethics AND (cloning OR reproductive techniques)
(ethic* OR moral*) AND (bioengineering OR cloning)
Keywords vs Subjects
To find subject headings for your topic:
Look to see if the database has an online thesaurus to browse for subjects that match your topic (check the Help screens).
Some databases publish thesauri in print (e.g. Thesaurus of Psychological Index Terms for the PsycInfo database).
Check the Library catalogue for subject headings used to describe the item.
Another way to find subject headings:
Start with a keyword search, using words/phrases that describe your topic.
Browse the results; choose 2 or 3 that are relevant.
Look at the Subject or Descriptor field and note the terms used (write them down).
Redo your search using those terms.
Your results will be more precise than your initial keyword search.
What are Subject Headings and Keywords?
Subject headings describe the content of each item in a database. Use these headings to find relevant items on the same topic. Searching by subject headings (a.k.a. descriptors) is the most precise way to search article databases.
It is not easy to guess which subject headings are used in a given database. For example, the phone book's Yellow Pages use subject headings. If you look for "Movie Theatres" you will find nothing, as they are listed under the subject heading "Theatres - Movies."
Keyword searching is how you typically search web search engines. Think of important words or phrases and type them in to get results.
Here are some key points about each type of search:
natural language words describing your topic - good to start with
pre-defined "controlled vocabulary" words used to describe the content of each item (book, journal article) in a database
more flexible to search by - can combine together in many ways
less flexible to search by - need to know the exact controlled vocabulary term
database looks for keywords anywhere in the record - not necessarily connected together
database looks for subjects only in the subject heading or descriptor field, where the most relevant words appear
may yield too many or too few results
if too many results - also uses subheadings to focus on one aspect of the broader subject
may yield many irrelevant results
results usually very relevant to the topic
Thanks to MIT Libraries for the use of their content: MIT Libguide: http://libguides.mit.edu/c.php?g=175963&p=1158594