Once the question has been developed the next step is to perform a comprehensive literature search, which aims to identify all relevant literature from a variety of sources.
Sources to search:
Google Scholar and Google Advanced Search can be useful when searching to find information that is not indexed in databases. However, these should be used to supplement comprehensive searches across scholarly databases, and not instead of them. There are limitations to using these resources:
You can also use a range of search techniques to help you to focus your searches across Google's resources.
The databases used may vary widely depending on the research question you are trying to find evidence to answer. However here are some good starting points for various disciplines listed below. Note that these recommendations are databases used commonly in systematic reviews in these disciplines, and your project may well have different requirements.
Any health or medical topic:
Psychology: PsycINFO, MEDLINE, Embase, Web of Science.
Agriculture: Web of Science, Scopus. Google Scholar could be added to increase comprehensiveness. You might also consider also searching CSIRO Online Journals if your topic has an Australian focus.
Education: ERIC, Education Database, A+ Education, PsycINFO.
Environmental science: Web of Science, Scopus, GreenFILE. Google Scholar could be added to increase comprehensiveness.
Criminology and justice: Criminal Justice Abstracts, NJCRS (National Criminal Justice Reference Service), PsycINFO.
Business, management, accounting: Web of Science, ABI/INFORM, Business Source Complete.
Social sciences: The social sciences are very diverse, so the databases for each project may be quite different. Some databases you could explore might be: Sociological Abstracts, Social Science Database, Scopus, Social Science Research Network (SSRN).
Other disciplines: See the tips for finding relevant databases below.
An excellent way to select databases is to find other systematic reviews on a similar topic to yours and see which databases they have searched
Library subject guides provide recommended discipline specific information sources that will be useful
UQ Library's Database search allows you to find databases appropriate for your research topic
The technical supplement to the Cochrane Handbook has a large list of databases for health sciences related topics
UQ Librarians can advise on appropriate databases for your particular research question
There is no set number of resources to search, only that the search is expansive and comprehensive. Best practice however is to search more than one resource. Systematic reviews in health and medicine will often search 3-5 databases, and searching more is not uncommon.
Grey literature is material that has not been made available via a commercial publisher. It’s important to consider whether you’ll need to search grey literature sources for your project. The main consideration is whether you think that the evidence you need to find for your review might actually be found in these types of sources, which will vary widely between research topics and disciplines. For some topics for example online reports might be a key source of evidence, and for others the evidence might only be reasonably expected to be found in published journal articles.
You should consider the requirements of your individual project carefully. Looking at other systematic reviews similar to your proposed review to see what grey literature sources they have searched can be helpful.
|Source||If you think that relevant evidence for your project might possibly be found in this type of material, these sources are good starting points:||If this type of material is particularly important for your project you might want to search extra sources:|
|Conference proceedings||Web of Science, Scopus||See some other sources in our Grey Literature guide|
Relevant for health disciplines. Usually clinical trials that have been registered, but not yet published as journal articles
|WHO ICTRP and ClinicalTrials.gov trials registries. These are the two sources recommended in the Cochrane Handbook||Some other registries are listed in our Clinical Guidelines and Trials guide|
|Theses and dissertations||ProQuest Dissertations & Theses Global||See some other sources in our Theses guide|
Includes government reports, technical reports, policy documents, clinical guidelines etc.
Google and Google Advanced Search. Many reports these days are made available on the web. See our information on searching Google for grey literature
Other search engines such as Bing. See our information on finding technical reports. Other relevant sources will vary significantly depending on the discipline or topic.
If there are organisations particularly pertinent to your topic (NGOs, government agencies, companies etc.) you might also go directly to their websites to search for material
Scholarly papers that have not yet undergone peer review or been published in a journal (this may make them inappropriate for many projects)
|These can be found in preprint servers for each discipline, eg. medRxiv and bioRxiv for the health sciences, EarthArXiv for environmental sciences etc. Google may help you identify the best preprint servers for your topic||You might try to identify extra preprint servers beyond the major ones for your topic|
Create a plan on searching grey literature to decide upon which resources will be used.
Report the searches according to PRISMA Guidelines: Rethlefsen, ML, Kirtley, S, Waffenschmidt, S, et al. PRISMA-S: an extension to the PRISMA Statement for Reporting Literature Searches in Systematic Reviews. Syst Rev. 2021;10(1):39.
Selecting and searching many of these resources in a systematic, reproducible manner can be tricky. The UQ Librarians can advise. For more in-depth information on grey literature see our Grey Literature guide.
Hand searching means looking through particular journals manually for relevant articles. The principle behind it is that not everything in a journal is indexed in bibliographic databases, and some journals are not covered by these databases. With online journals hand searching is easier in that the journal can be searched with keywords and advanced search tools.
Hand searching should be considered if you suspect that relevant evidence from journals is not being retrieved in your database searches, e.g. you become aware that a particularly relevant journal is not covered adequately by the databases. Hand searching can be slow and time-consuming and will not be required for many projects. For example it is not required for Cochrane Reviews, and Vasser et al.  found that a very low percentage of published systematic reviews reported doing hand searching. A review by Cooper et al.  provides a very good summary of the evidence around hand searching.
Snowballing (sometimes called reference checking) is the process of examining the references of known documents to identify other relevant literature. In a systematic review it is best practice to check the references of all included studies, so this is typically done after full text screening is completed. The value in this process is to help identify references which may lie outside of traditional indexing resources. Citation databases such as Web of Science and Scopus may be helpful in examining the references of particular papers, or reference lists can be examined manually.
Related to snowballing, forward citation checking means to check the studies that have cited your included studies, again to identify other relevant literature. Forward citation checking is less commonly done than snowballing  but is a relatively undemanding way to increase the thoroughness of your project. Citation databases such as Web of Science, Scopus or Google Scholar can be the easiest way to identify papers that have cited a particular paper.
1. Vassar, M, Atakpo, P, Kash, MJ. Manual search approaches used by systematic reviewers in dermatology. J Med Lib Assoc. 2016;104(4):302-304.
2. Cooper, C, Booth, A, Britten, N, Garside, R. A comparison of results of empirical studies of supplementary search techniques and recommendations in review methodology handbooks: a methodological review. Syst Rev. 2017;6(1):234.
3. Briscoe, S, Bethel, A, Rogers, M. Conduct and reporting of citation searching in Cochrane systematic reviews: a cross-sectional study. Res Synth Methods. 2020;11(2):169-180.