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Systematic Reviews: Search Strategy

Sensitivity vs precision

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.

  • Sensitivity – the number of relevant results identified divided by the total number of relevant results in existence.
  • Precision - the number of relevant results identified divided by the total number of results identified.

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:

  • All concepts are included in the strategy
  • All appropriate subject headings are used
  • Appropriate use of explosion
  • Appropriate use of subheadings and floating subheadings
  • Use of natural language (text words) in addition to controlled vocabulary terms
  • Use of appropriate synonyms, acronyms, etc.
  • Truncation and spelling variation as appropriate
  • Appropriate use of limits such as language, years, etc.
  • Field searching, publication type, author, etc.
  • Boolean operators used appropriately
  • Line errors: when searches are combined using line numbers, be sure the numbers refer to the searches intended
  • Check indexing of relevant articles
  • Search strategy adapted as needed for multiple databases

Planning your search - what to include

Where to search

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:

Why Should You Use Grey Literature?

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 6.2.1.8). 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].