Systematic reviews aim to locate, select and appraise as much as possible of the texts relevant to the particular review question(s). There 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):  

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 (2005, p. 1065) conclude that a,

"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 Peacock, 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.