Systematic reviews aim to locate, select and appraise as much as possible of the texts relevant to the particular review question(s).  The are numerous approaches used to locate studies and the effectiveness of these are influenced by the question being addressed and the nature of the field.  Take a look at the table below produced by Greenhalgh and Peacock (2005),  

Search Approach	Number of Relevant articles Located
Protocol driven (search strategy defined at the outset of the study)	150
Electronic database search*	126
Hand search (32 journals)	24
"Snowballing" (emerging as the study unfolds)	252
Reference tracking - scanning the reference lists of all full text papers	218
Citation tracking: using special citation tracking databases to identify articles that had subsequently cited those papers	34
Personal knowledge (what you know and who you know)	119
Existing knowledge and resources	85
Personal contacts and academic networks	29
Serendipitous discovery (such as finding a relevant paper when looking for something else).	5
Total in final report	495

It is clear that a search needs to incoporate a combination of protocol driven, snowballing and personal knowledge approaches.  A search that is restricted to database searching alone is likely to locate a very small percentage of relevant articles.  Greenhalgh and Peacock (p. 1065) conclude that,

"systematic review of complex evidence cannot rely solely on predefined, protocol driven search strategies, no matter how many databases are searched. Strategies that might seem less efficient (such as browsing library shelves, asking colleagues, pursuing references that look interesting, and simply being alert to serendipitous discovery) may have a better yield per hour spent and are likely to identify important sources that would otherwise be missed." 

To ensure a thorough search, we recommend a six-step search strategy.  To access guidance on the six steps click on the links below:

• Step 1: Create a bibliography database
• Step 2: Protocol driven search
• Step 3: Snowballing
• Step 4: Personal knowledge
• Step 5: Search for 'grey' literature
• Step 6: Keep up-to-date

Reference:
Greenhalgh, T. and Peacok, R. (2005), Effectiveness and efficiency of search methods in systematic reviews of complex evidence: audit of primary sources.  British Medical Journal. November 5; 331(7524): 1064–1065.

 

The text on this page was created by Professor David Denyer, Professor of Organizational Change, Cranfield School of Management.