Evidence-based practice: Acquire (Search)



Sources of evidence

The evidence you find to answer your clinical question will consist of both primary and secondary sources.  Primary sources are journal articles where the researchers report on the methods and outcomes of a single study.  In EBP secondary sources are sources that report on the results of multiple studies to answer a clinical question, for example systematic reviews and clinical guidelines.  You can search for both primary research studies, systematic reviews and clinical guidelines in the databases listed below.

Plan your search

Before you begin searching it is helpful to plan out your search. This is an important step between developing your answerable research question  and searching for literature. 

Download this 'Planning your Search: Using PICO' document and use it to:

  • document your research question 
  • identify keywords for each PICO element 
  • note down search terms and phrases including alternative/ synonyms
  • document a search strategy 
  • note any limiters 

The document includes a template and an example as a guide. 

Searching with Keywords

The three most commonly used operators are ANDOR and NOT.   They can be used to broaden or narrow your set of results and to exclude unwanted search terms and concepts.

For example:

If you want to find information on how do violent video games affect children and their brain development

AND will narrow your search returning results that contain all of your search terms 

violent video games AND children AND brain development

OR will broaden your search returning results that contain any but not all of your search terms. It is useful for finding synonyms or where different words are of equal value in your search 

kid OR kids OR child OR child OR children OR schoolchildren OR schoolchild OR school child OR school children

video games OR video game OR video gaming

NOT will narrow your search by eliminating words from your search results. It should be used with care as it can easily exclude relevant results.

behavioural problems NOT behavioural therapy

To search for two or more words in exact order, place double quotation marks " " around the words. The database will only return articles containing that specific phrase rather than articles containing each word found individually anywhere in the text.


The phrase "video gaming" will retrieve articles with all words as you typed them in with no other words in between. 

Truncation also known as stemming, is a technique that broadens your search to include alternative word endings and spellings.

To use truncation, enter the root of a word and put the truncation symbol * at the end.
The database will return results that include any ending of that root word.
For example:
child* = child, child's, children, children's, childhood

counsel* will retrieve both spellings = British English counselling  and  American English counseling

It is important not to shorten the root too much as it may retrieve too many irrelevant results. For example chil*  will bring up childless, chiller, chilly, and Chile.


Wildcard symbols are used to represent one character or letter inside of a search term. This search technique useful if a word is spelt in different ways but still has the same meaning. 

Common symbols include: 

(exclamation mark)

? (question mark), and

# (number sign)


behavo!r = behaviour, behavior
p?ediatric = pediatric, paediatric

The symbols vary between databases so it is recommended to check the Help section.


Controlled Vocabulary - Subject Term Search

Controlled vocabulary, is a standard set of terms that describe specific concepts covered in the article. When you search using subject terms you are searching for matching the content of the database subject field rather than searching through the text as it happens with a keyword search. For example, the PsycINFO database uses  'APA Thesaurus of Psychological Index Terms', the Medline database refers to MeSH (Medical Subject Headings), and CINAHL database has CINAHL Subject Headings. 

Hierarchies of Evidence

The aim of EBP is to find the best available evidence to answer a specific question.  Researchers have developed evidence hierarchies,  also known as Levels of Evidence that rank study designs according to the probability of bias. In looking for the best available evidence researchers start with the study designs at the top of the hierarchy, for example for intervention questions, systematic reviews of randomized  controlled trials.  If no high level studies are available the researcher must search for evidence at a lower level.

Note, there are different hierarchies of evidence for different question types. See the links and references below for examples and more information about hierarchies of evidence. 


Hoffmann, T., Bennett, S., & Mar, C. D. (2016). Evidence-based practice across the health professions (3rd ed.). Chatswood, Australia: Elsevier Australia.

ISU Libraries. (2015).  Evidence-based practice, step 2 (part II): Searching- levels of evidence [video] https://youtu.be/tqQ94s-3dCc

National Health and Medical Research Council (2009). NHMRC levels of evidence and grades for recommendations for developers of guidelines https://www.nhmrc.gov.au/sites/default/files/images/NHMRC%20Levels%20and%20Grades%20(2009).pdf

See Table 3 NHMRC Evidence Hierarchy: designations of ‘levels of evidence’ according to type of research question (p.15)

Oxford Centre for Evidence -Based Medicine Levels of Evidence - Table 

OCEBM Levels of Evidence Working Group*. "The Oxford 2011 Levels of Evidence". Oxford Centre for Evidence-Based Medicine. https://www.cebm.net/wp-content/uploads/2014/06/CEBM-Levels-of-Evidence-2.1.pdf

Oxford Centre for Evidence -Based Medicine Levels of Evidence - Introductory document

Howick, J., Chalmers, I., Glasziou, P., Greenhalgh,  T., Heneghan, C., Liberati, A., Moschetti, I., Phillips, B., & Hazel Thornton, H. (2011).  "The 2011 Oxford CEBM evidence levels of evidence (Introductory Document)". Oxford Centre for Evidence-Based Medicine. https://www.cebm.net/wp-content/uploads/2014/06/CEBM-Levels-of-Evidence-Introduction-2.1.pdf

This document provides a guide to using the OCEBM (2011) Levels of Evidence Table.