The goal of systematic review searches is to identify all relevant studies on a topic. Therefore, systematic review searches are typically quite extensive. It is necessary, however, to strike a balance between striving for comprehensiveness and maintaining relevance when developing a search strategy. Increasing the comprehensiveness (or sensitivity) of a search will reduce its precision and will retrieve more non-relevant articles.
For more information, check out IOM Standards for Systematic Reviews: Standard 3.1: Conduct a comprehensive systematic search for evidence
The goal of a systematic review search is to maximize recall and precision while keeping results manageable. Recall (sensitivity) is defined as the number of relevant reports identified divided by the total number of reports in existence. Precision (specificity) is defined as the number of relevant reports identified divided by the total number of reports identified.
Issues to consider when creating a systematic review search:
Cochrane Handbook: Searching for Studies (See Part 2, Chapter 6)
Search Strategy Checklist (Duke University)
Peer Review of Electronic Search Strategies (PRESS) (CADTH, Canada)
Systematic Review Information Retrieval Checklist (Campbell Collaboration)
Literature Searching Guidelines Checklist for Researchers (Yale University)