Deciding which bibliographic sources to search is crucial to the quality of the review. For most Systematic Reviews all available relevant databases would be used to ensure comprehensiveness. Do you know which subject specific databases are relevant to your subject? Are you planning on including grey literature?
Finding subject specific databases for your subject:
Subject specialist databases
Searching within a subject specialist database such as Medline, British Education Index or Business Source Ultimate will immediately focus your search, helping you more easily identify key references, publications or authors within your discipline. You will also find references which are not covered by the interdisciplinary databases.
Subject specialist databases usually include more advanced searching features, such as the option to choose subject headings from a thesaurus, or use a controlled vocabulary. This allows you to search more successfully, using subject-specific terminology which may have a very different meaning in other disciplines.
Interdisciplinary databases
Using databases such as Scopus and/or Web of Science allows you to broaden your search to find a range of peer-reviewed content related to your topic. They are also very useful if your research topic is interdisciplinary: e.g. Medical Education.
Unpublished material is called grey literature and can contain lots of different types of documents, including conference proceedings, theses, government reports, technical reports, official documents and even blog posts, tweets etc. You would normally expect a systematic review search to include grey literature, so that you don’t just retrieve the research that has generated ‘positive results’ which would give your research a bias. For example, if an intervention / drug / treatment / method has seen positive results, authors are more likely to submit the research to a journal and it is also more likely to be accepted as it is seen as interesting. Less positive or notable results may not be published and therefore become classed as grey literature. However, this grey literature is still important to identify in order to get a balanced view and ensure that your research is systematic and not just a literature review.
If you are not sure whether you should be searching grey literature as part of your systematic review, then please discuss this with your supervisor. The scope could be to search only formally published literature but this must be made clear at the start.
Use the Grey Literature Guide to find out which sources you should be checking, linking you to our Guides on conference proceedings, theses and government reports.
Using Google Scholar to find journal articles for systematic reviews comes with major warnings. It is certainly not sufficient to be used on its own to support systematic review searching and if it is used, it definitely needs to be used in conjunction with the Library databases already mentioned. It also may not be appropriate to use Google Scholar at all, as it is very hard to replicate the search with any accuracy. Unlike a database which has been indexed in a systematic way, Google will not reveal how its algorithms and rankings work and therefore, there is no certainty that the results that are generated one day will be generated another, even if the same keywords are used. As a result, Google Scholar may be used at the scoping search stage, but it is rarely used further on.
If researchers need to find grey literature using the internet, they often choose to use Google advanced search as they can have more control over the search and it can be more easily replicated. See the Grey Literature Guide.