A systematic search to identify studies must be comprehensive and it must strike a balance between recall and precision. In other words, don't expect to retrieve only relevant articles.
Most studies used in the review will be identified using electronic databases (e.g. PubMed). But, The Cochrane Collaboration states that "efforts should be made to identify unpublished studies." (Cochrane Handbook 6.2.4) Librarians can help to identify sources for finding grey literature.
Expect a large number of results. Depending on the scope of your topic, your search may result in as few as a couple hundred or as many as several thousand articles.
Some key steps in searching for studies include:
- Identifying databases to be searched.
- Identifying search terminology.
- Constructing and running database searches.
- Conducting hand searches of specialized journals.
- Searching reference lists of relevant studies.
- Contacting recognized experts working in the field.
- Searching relevant grey literature sources (e.g. clinical trials registers, conference proceedings).
It’s important to note that the National Academies of Sciences, Engineering and Medicine recommends working “with a librarian or other information specialist trained in performing systematic reviews to plan the search strategy” (Standard 3.1.1).