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Evidence Based Medicine: Keywords

Introduction to fundamental concepts of evidence based medicine and practice.

Simple keyword searching

1. Identify concepts for searching

Before you even start, think about the concepts you want to find in the research papers in the database.

Concepts are the essential ideas or elements in a sentence or question.

When searching a database to find information related to some research question, begin by looking at your question and identifying the concepts in the query. In EBM, these concepts are usually be the elements of your PICO. For example:

Clinical query: Does smoking cause headaches in teenagers?
Concepts: smoking, headaches, teenagers

2. Find synonyms for your concepts

Synonyms are words that have identical or related meanings.

Authors of research papers may use different words to describe the same concept. Consider each of the concepts from your research question, and list different words or phrases for each one.

smoking headaches teenagers
smoking headaches teenage
cigarettes head pains teenagers
young adults

3. Phrasing and truncating of your synonyms

Use PHRASING to find concepts that are expressed using more than one word.

Many concepts can expressed as a combination of words rather than a single word, e.g. young adult. However, simply searching for young adult, etc. will find papers that contain the words young and adult regardless of whether they occur together as the phrase young adult or whether they occur in different sentences. Phrasing or phrase searching ensures that the database searches for your search terms as you have written them. To search for the exact term young adult, use quotation marks, e.g. "young adult".

Remember to allow for variations in phrases, e.g. "young adult", "young adults", "young adulthood".

Applying phrasing to our list of terms:

smoking headaches teenagers
smoking headaches teenage
cigarettes "head pain" teenagers
"head pains" adolescents
"young adult"
"young adults"
"young adulthood"


Use TRUNCATION to search for words with several possible endings.

When the search term consists of a single word, you can search for all of these at once by truncating (trimming) them all back to their common element, then adding the truncation symbol: this is usually an asterisk (*), e.g. in PubMed, or a dollar ($) sign.

E.g. teenagers truncates to teenage*. PubMed will reconize this and search for all terms beginning with teenage, including teenage, teenaged, teenager, and teenagers.

Applying truncation to our list of terms:

smoking headaches teenagers
smoking headache* teenage*
cigarette* "head pain" adolescen*
"head pains" "young adult"
"young adults"
"young adulthood"


  1. Consider all the variations you might get and whether they are appropriate. E.g. searching for child* will not only find papers containing the words child, children, and childhood, but also those containing the words childish, childishly, and childlike. 
  2. Don't trim too much off your common element. E.g. truncating to chil*, besides finding papers containing child, etc. will also find papers containing chillschilly, and Chile.
  3. PubMed will only search for the first 600 variations of the truncated term. This might seem like a lot, but sometimes it is not enough (see here for an example).


4. Combinging your search terms

Search terms can be combined in most databases using OR, AND, and NOT.

1. OR

Use OR to combine search terms when you want at least one of the terms to appear in a research paper

This is how the database is instructed to search for multiple synonyms for a single concept:

smoking smoking OR cigarette*
headaches headache* OR "head pain" OR "head pains"
teenagers teenag* OR adolescen* OR "young adult" OR "young adults" OR "young adulthood"

Combining synonyms with OR broadens (i.e. increases the number of papers found by) your search by finding results that contain at least one of the terms sought.


2. AND

Use AND to combine concepts when you want several concepts to appear in a research paper.

Use parentheses to group your terms for each concept, then place AND between the parentheses:

(smoking OR cigarette*) AND (headache* OR "head pain" OR "head pains" ) AND (teenag* OR adolescen* OR "young adult" OR "young adults" OR "young adulthood")

Combining conepts with AND narrows (i.e. decreases the number of papers found by) your search by requiring that all your concepts be in each result.


  1. If you forget to include AND, OR, or NOT, many databases, e.g. PubMed, assume you mean AND.


3. NOT

Use NOT to exclude terms from your search.

If you wanted to exclude papers that mention cigars from your search, you could add NOT cigar*:

(smoking OR cigarette*) AND (headache* OR "head pain" OR "head pains" ) AND (teenag* OR adolescen* OR "young adult" OR "young adults" OR "young adulthood") NOT cigar*


  1. Be very careful using NOT, as it will exclude research papers from your results based soley upon the occurrence of the excluded term. In the example above, papers that only discuss cigars will be exculded, but so will papers that focus on cigarettes and only mention cigars in passing.
  2. Some databases do not use NOT.