2. Identify synonyms and related terms

It is important not to overlook this stage in the search process. Time spent identifying all possible synonyms and related terms for each of your PICO elements or concepts will ensure that your search retrieves as many relevant records as possible.

It might be useful to check relevant dictionaries, encyclopedias and key texts for alternate terms.

Build a list of each of the search terms you identify. For example, if you were searching for:

Your list of synonyms and related terms might include:

3. Use truncation and wildcards

The definition of ‘truncation’ is to shorten or cut-off at the end. Truncation is used in database searches to ensure the retrieval of all possible variations of a search term. All databases allow truncation, but the symbols used may vary, so it is best to check the database help for details.

Databases usually allow words to be truncated either at the end, or internally:

• Truncating a word at the end ensures that all variations of the word, beginning with a specific root, will be retrieved. This is particularly useful for retrieving singular and plural versions of words.

Be careful not to truncate terms too early, or you may retrieve a high number of irrelevant documents.

Most databases use an asterisk (*) to find alternate endings for terms. For example:

Truncating a word internally ensures that any variations of spelling of a word can be retrieved. For example, pediatrics or paedetrics. It allows you to search for alternate spellings of words - extremely useful when searching for American and English spellings of words.

Internal truncation is available in some databases such as EMBASE.com. Unfortunately it is not possible in PubMed.

For example, a question mark included within a word can designate zero or one character in that place:

4. Combining terms
 

• Boolean Operators

Boolean operators (OR, AND, NOT) allow you to link terms together, either to widen a search or to exclude terms from your search results.

OR		Use to broaden your search, increasing the number of references retrieved. Use "OR" to search for synonyms and related terms for each concept within a research question. For example, when searching for the concept "exercise based rehabilitation" you might use the following terms: rehabilitation OR exercise OR exercise therapy OR sports OR exertion OR physical training OR aerobics OR kinesiotherapy
AND		Used to narrow a search, therefore decreasing the number of references. For example, searching for: coronary heart disease AND Asian Americans Would retrieve just those references covering both topics.
NOT		Used to narrow a search, therefore decreasing the number of references. For example: dogs NOT sheep Would retrieve references dealing with females, but not those which discuss males. Caution should be exercised when using NOT, In the example above, research dealing with both dogs and sheep would be excluded from the search results.

• Proximity Operators

Proximity (sometimes called “adjacency”) searching is similar to using Boolean operators in that you are specifying relationships between 2 or more terms. However, proximity searching allows you to specify the proximity of words to each other. Some databases allow you to search for words within a specified number of words from each other. This feature is not available in PubMed.

For example you can use NEAR/n or NEXT/n:

drug* NEAR/2 adverse  means the words must be within n words inclusive of each other in the record.

adverse NEXT/2 reaction*  means he words must be within n words inclusive of each other in the same order as they appear in the search form.

• Nesting

Nest search terms to control the logic of your search. For example:

(rehabilitation OR exercise OR exercise therapy OR sports OR physical training) AND (Coronary heart bypass OR myocardial ischemia OR myocardial infarction OR coronary disease OR coronary thrombosis)

5. Identify search limits / exclusion criteria

You should next think about the limits you intend to apply to your search.

6. Keyword vs subject searching

Controlled vocabularies (such as the MESH subject headings used in Medline and EMTREE subject headings used in EMBASE) provide an organised approach to the way knowledge is described. 

Their use is extremely important as they bring uniformity to the indexing of publications included within a database. Using the same terminology throughout a database creates consistency and precision and helps you to find relevant information no matter what terminology the author may have used within their publication.

Indexing is usually a manual process. Databases such as MEDLINE employ specially trained indexers to read the full-text of each publication then identify all of the concepts covered within the article. These concepts are then translated to the controlled vocabulary used within the database. It is the indexer’s job to ensure that each concept included in the article are identified and assigned a term.

N.B. Text word or Keyword searches are extremely important when conducting systematic reviews, and should be used in combination with the relevant controlled vocabularies or subject headings within each of your database searches:

• Authors may not describe their methods or objectives well and indexers are not always experts in the subject areas
• The available indexing terms might not correspond to the terms the searcher wishes to use. Cochrane Handbook for Systematic Reviews of Interventions (2008, p. 130)

Each database may use different subject headings to describe the same concept. As an example, the term “complementary medicine”:

The MEDLINE (PubMed/OVID), Embase.com, and CINAHL databases provide a search option to “explode” terms. PubMed automatically explodes terms, although there is the option of choosing not to explode a term. Exploded searches retrieve indexed records for a term, plus other terms which are a derivative (more specific, narrower terms) of the search term. Exploding search terms provides a fast way to find related concepts in a single search.

For example, if a search for "complementary therapies" in MEDLINE was exploded: