The number of databases used in a systematic review should generally be at least three to four, and often more, chosen deliberately to cover the relevant literature rather than to hit an arbitrary total. There is no single fixed rule that fits every discipline, but reputable guidance is clear that searching only one database is not enough for a defensible systematic review, because no single database indexes all the studies you need. The right number is the smallest set that gives you comprehensive, unbiased coverage of your topic.
In practice, most health and social science reviews search a combination of major bibliographic databases plus supplementary sources. The guiding principle is comprehensiveness: you are trying to minimise the chance of missing eligible studies, which is what separates a systematic review from an ordinary literature review.
Why one database is never enough
Each database indexes a different, overlapping slice of the published record. A study that appears in one may be entirely absent from another, and the overlap between even the largest databases is far from complete. Relying on a single source therefore introduces selection bias before you have screened a single title. Because a systematic review aims to identify all the evidence that meets your inclusion criteria, searching multiple databases is a methodological requirement, not a matter of preference.
This is why established standards such as the Cochrane Handbook and the reporting guidance in PRISMA expect authors to search several sources and to document exactly which ones, with dates and result counts, so the search can be reproduced and appraised.
Which databases to choose
The correct databases depend on your discipline, so choose by relevance rather than by habit. Common choices include:
- Health and medicine: MEDLINE (often via PubMed or Ovid), Embase, CINAHL and the Cochrane Library.
- Psychology and behavioural sciences: PsycINFO alongside the medical databases.
- Nursing and allied health: CINAHL, MEDLINE and Embase.
- Social sciences and education: Scopus, Web of Science, ERIC and Sociological Abstracts.
- Multidisciplinary coverage: Scopus and Web of Science, which span many fields and help catch studies outside a single subject index.
A frequently used pattern in health research is to combine two or three subject specific databases with one or two multidisciplinary ones. What matters is that your selection can be justified: you should be able to explain why each database is appropriate for your question and why the set as a whole covers the field.
Beyond database searching
Databases alone rarely capture everything, so a rigorous review supplements them with additional search methods. These include:
- Reference list checking, sometimes called backward citation searching, where you scan the bibliographies of included studies.
- Forward citation searching, where you find newer papers that cite a key study.
- Grey literature searching, covering theses, government reports, conference proceedings and trial registries, to reduce publication bias.
- Hand searching key journals and contacting experts in the field.
Grey literature matters because studies with negative or inconclusive results are less likely to be formally published, and ignoring them can skew your conclusions. Trial registries and dissertation repositories help address this gap.
Quality over quantity
It is tempting to think that more databases always mean a better review, but the goal is appropriate coverage, not the largest possible count. Searching ten poorly chosen databases with a weak search string is worse than searching four well chosen ones with a carefully built, peer reviewed strategy. The strength of your search rests on three things together: the databases you select, the search terms and Boolean logic you use, and the transparency with which you report both. A well designed strategy tailored to each database interface will usually outperform a scattergun approach.
You should also record everything as you go: the database name and platform, the date searched, the full search string and the number of results. This documentation feeds directly into your PRISMA flow diagram and lets examiners and reviewers verify your work.
Guidance for Australian postgraduate students
For students at Australian universities, the safest course is to check both your discipline norms and your university library guidance, since faculties and supervisors often specify a minimum number of databases for Higher Degree by Research work. Most Australian university libraries publish detailed systematic review guides and offer consultations with research librarians who can help you match databases to your question and refine your search string. Using that support early tends to save weeks of rework later.
If you are planning or writing up the search and screening stages, our literature review writing help service can support you in building and documenting a reproducible strategy, and for a full research project our dissertation writing help covers how the review fits into the wider methodology.
In summary, there is no magic number, but three to four well chosen databases is a common and defensible minimum, with more added when the topic spans several fields. Combine those databases with reference checking, citation searching and grey literature, document every step, and choose sources by relevance rather than convenience. Doing so gives your systematic review the comprehensive, transparent foundation that reviewers and examiners expect.