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Rules of Thumb

These are common rules of databases. Your database may not follow them.

  • Use parentheses to group complicated search requests.
  • Use quotation marks to group phrases.
  • The most common truncation symbols are * and ?
  • Most databases use AND as a default Boolean term.
  • Capitalize Boolean terms.


Boolean Operators

Most databases allow the user different searching methods. One of the most common searching methods is Boolean Searching, also called keyword searching. This type of search tells the database to retrieve all of the records in the database which contain a word or a set of words. You can alter the results by using Boolean Operators which are the words AND, OR and NOT. See below for an explanation of these terms. Some databases require the Boolean operators to be capitalized or they are searched just like regular search terms.

Example: cookies AND milk

Will retrieve records which contain the word “cookies” and the word “milk.” This operator is used to lessen the number of records retrieved. This is the most common default Boolean term.

Example: caffeine OR coffee

Will retrieve records which contain the word “caffeine” or the word “coffee.” This operator is used to broaden the number of records retrieved.

Example: chocolate NOT cake

Will retrieve records which contain only the word "chocolate" but not the word "cake." This operator is used to reduce the number of records retrieved.


Use to preserve the “logic” of your Boolean Search. Nesting is the use of parenthesis to put your search words into sets. If you do not use parentheses, Boolean terms are connected according to the default functions of the database. Because it is difficult to keep track of differences in databases and because almost every database accepts parentheses, it is suggested that parentheses ALWAYS be used in a complicated search phrase.

(Huntingtons AND disease) OR chorea    

Huntingtons AND (disease OR chorea)

((diabetes OR diabete) AND (hypertension OR (high blood pressure))) NOT therapy


Use to find different forms of words in a Boolean or keyword search. Some databases use the asterisk, some use a dollar sign, and others use the question mark. The symbol may represent one character or they could represent multiple characters. They usually apply to word endings. They may or may not apply at the beginning or middle of a word. Check the help function of the database you are using to learn the truncation symbol and rules.

Neuron*     Will retrieve neuron, neurons, neuronal, neuronopathy, etc.

The most common truncation symbols are * and ?


Commonly used words that occur too frequently in records will either be ignored and not searched or they will automatically stop or prevent a computer keyword search. Stop words are usually listed in the Help screens of whatever database you are using. Commonly used words rarely help help you narrow down your search results.

Some common stop words are: the, an, at, for, from, of, then.

Phrase Searching

Phrases are treated differently in databases. Some automatically assume two adjacent words are a phrase. Others require the use of quotation marks or parentheses to search for a phrase. Databases that automatically assume two words are a phrase often ignore the quotation marks if they are unnecessary. Because it is difficult to keep track of differences in databases, it is suggested you use quotation marks when you enter a phrase.

"common cold"

"shortness of breath"      (View Stopwords to avoid a possible
                                                 pitfall from "of.")

An exact phrase finds the words in exactly the same order.

“Heart attack” 

“attack heart”

Subject Headings

Most databases assign subject categories in a hierarchy from general to specific. 
Medical Subject Headings (MeSH) are hierarchical. To Explode a Subject Heading
in a search includes all subject headings subordinate to the Subject Search Term.
 §  Dogs
o    Terrier
§  Fox terrier
§  Bull terrier
o    Hound
§  Bassett hound
§  Blood hound
A search for the following term in MEDLINE retrieves the following results.  
Terrier                          matches records with Terrier as a subject heading

 Terrier (Exploded)      matches records with Terrier, Fox terrier, and Bull terrier
                                 as subject headings

Subject headings are VERY important in searching health science databases.
CINAHL and MEDLINE are specifically designed to be searched using the
subject headings.


Many databases contain a thesaurus. This is a directory of assigned Subject
Headings {eg. Medical Subject Headings (MeSH)}. Searching for a subject
heading instead of a word that happens to appear anywhere in a record can
reduce the number of irrelevant records retrieved from your search.

Some databases will automatically include synonyms in the search, whether you
want them to appear or not, so check the rules. This is called thesaurus mapping.

 cancer       Finds cancer and neoplasm in the PubMed database when thesaurus mapping occurs.


The Index is the list of words used by all the records in a database. A database does not directly search its records but actually searches its Index for your word(s), which then tells the database which records contain those words. Some databases allow you to browse the Index directly. The PubMed database contains four separate indexes: a Phrase Index, a Journal Title Index, an Author Names Index, and MeSH Headings.

Stopwords are not included in the database. That is why they cannot be searched.

Subject Guide

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Shelley Arvin
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