Usually in systematic reviews a researcher will search enough resources to be able to state with confidence that the literature for the subject area has been comprehensively searched. However, there are no rules about how many resources should be included. Often the best test is to search a new resource for a limited component of a review to see if new items are identified.
The scope of your research question should be kept narrow so that a large number of irrelevant items do not have to be excluded from your result. If you are getting more than 3,000 items from the first database used with the search strategy for the review the scope of the research question or the structure of the search strategy should be examined closely.
The search engines to include in a systematic review will vary depending on the subject field and question being asked.
Several suggestions for databases to search are listed below. But we recommend you consult the Subject Research Guide in your field for a more extensive list. See http://unimelb.libguides.com/
Do not search both Medline and PubMed. Medline (Ovid) searches are equivalent to searching all of PubMed and PubMed has limited proximity searching which can be important for reducing the search results found in systematic review searching.
Extensive systematic reviews can include Web of Science Core Collection and/or Scopus. They may be used to follow up references and related articles of key included papers. Google Scholar may be used for a simplified one line version of the search strategy however its use is often restricted to a few pages of results.
Most databases offer a thesaurus or list of available subject headings that can be allocated to an article by the author or indexer.
Use of the preferred term is a powerful and controlled way of directly accessing most if not all of the material within a field of study.The lists of preferred terms in Medline (MeSH), Embase (Emtree)) and Psycinfo are particularly extensive. Each is a hierarchical arrangement of broader terms, preferred and related terms, and narrower terms, designed to map the context and content of their respective fields.
In the OVID versions of Medline, Embase and Psycinfo the Search Tools feature offers a Map Term function for looking up the preferred term(s) for a given topic.
Take the term Acquired Brain Injury. You can map this to the preferred terms(s); check the definition of the preferred term in its scope notes; and "explode" to include all narrower terms.
The Medline MeSH tree is as follows. The preferred term is Brain Injuries, but this can be further subdivided.
The syntax to retrieve all uses of the term brain injuries AND its narrower terms is...
Additional search limits - or filters - can be applied, such as limiting to English language, human or animal, gender, age group, whether peer reviewed, and type of study (such as review, therapy, diagnosis, prognosis, or causation-etiology).
The Emtree headings in Embase have a different arrangement. Acquired Brain Injury is a narrower term within Brain Injury.
Psycinfo has a different tree again, and Traumatic Brain Injury is the preferred term.
Boolean searching looks for the presence or absence of words in the fields of a record, or the text of a document.
Proximity searching allows better control of the relevance of concepts by adjusting their proximity to one another. If the concepts occur close together in a sentence or paragraph, the topics are more likely to be relevant than if they are widely separated.
Most database platforms offer proximity operators to specify word order and separation. Check the help system of the database you are using and look for proximity to find information on how to apply it correctly.
Replace # with the maximum number of words to occur between the two concepts.
Grey literature is unpublished material or has been distributed outside mainstream commercial publishing. It may include reports, theses, government and NGO publications, conference abstracts and proceedings, registries of clinical trials and prospective studies, and the results of hand searching or corresponding directly with authors.
The Grey Literature for Health Sciences and Medicine LibGuide includes information and links for grey literature sources including sections:
What is grey literature?
Finding grey literature
Policies and Reports
Health and Medicine sources