The goal of systematic review searches is to identify all relevant studies on a topic. Systematic review searches are therefore typically quite extensive. However, it may be necessary to strike a balance between the sensitivity and precision of your search.
Increasing the comprehensiveness of a search will reduce its precision and will retrieve more non-relevant results. However, ... at a conservatively-estimated reading rate of two abstracts per minute, the results of a database search can be ‘scanread’ at the rate of 120 per hour (or approximately 1000 over an 8-hour period), so the high yield and low precision associated with systematic review searching is not as daunting as it might at first appear in comparison with the total time to be invested in the review. (Cochrane Handbook for Systematic Reviews of Interventions, 2008, Section 6.4.4)
See also: IOM Standards for Systematic Reviews: Standard 3.1: Conduct a comprehensive systematic search for evidence
Issues to consider when creating a systematic review search:
Other registries are available and may be worth consulting depending on your subject.
The search strategy must be comprehensive, hence a number of scholarly databases should be searched. Which databases you choose will depend on the topic of the review.
The databases listed below are usually the key databases to search for any systematic review:
The term grey literature "is usually understood to mean literature that is not formally published in sources such as books or journal articles" (Lefebvre et al. 2011, section 188.8.131.52). Grey literature may include research reports, conference papers, dissertations and theses, clinical trials, government documents, census data, standards, patents, and other research outputs.
Grey literature has traditionally been considered somewhat difficult to locate, but it is important to consult these unpublished studies to reduce the risk of publication bias in results.
Reference: Lefebvre, C., Manheimer, E. and Glanville, J. (2011) 'Searching for studies' in Higgins, J. P. T. and Green, S., eds., Cochrane handbook for systematic reviews of interventions [online], Version 5.1.0, available: http://handbook.cochrane.org/ [accessed 19 Dec 2014].