About Systematic Reviews

How Many Studies Should Be
Included in a Systematic Review?

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A systematic review is a study of studies. Because of the nature of a systematic review, it is easy to assume a large number of studies are required to validate the assessment. However, there are well-founded systematic reviews that include no studies at all.

Generally, you’d want to appraise and synthesize two to three studies for a sound systematic review, especially if the topic has an adequate amount of existing literature. However, there is no set minimum number of studies to include in a systematic review. There’s also no maximum number of studies that is too much to include. The more studies you examine, the more informative your research will be.

It is important to note that high-quality systematic reviews and meta-analyses take great care to locate all relevant studies based on developed eligibility criteria, critically assess each study, synthesize data and present a balanced summary of the findings.

What Is the Minimum Number of Studies to Include in a Systematic Review?

There is no minimum number of studies to include in a systematic review. The number of studies you include in a systematic review largely depends on your research topic, as well as the amount of supportive evidence available. A systematic review may be valid even if it contains no studies, as it indicates the field’s lack of research and knowledge gaps.

The types of systematic reviews in research you’re undertaking can also impact the number of studies to include. Generally, you’ll want to aim to have at least two to three studies assessed with a systematic review critical appraisal tool if your research question deals with a topic that already has a good amount of available literature.

What Types of Data Can I Include in a Systematic Review?

Primary studies comprise the majority of systematic reviews. Types of primary studies include:

  • Abstracts
  • Journals
  • Trial registers
  • Clinical studies
  • Regulatory reviews
  • Participant data

Can Other Systematic Reviews Be Included as Data?

Since systematic reviews are considered secondary studies, including them as data is a grey area. However, there’s a consensus that they should be noted in order to express that there have been reviews done on the topic. Similarly, you may not want to include a systematic review in a scoping review. Instead, you can include the relevant studies the review has considered.

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Where to Find Studies for a Systematic Review

You can find studies for a systematic review from various databases. There are several large ones that have become a necessity for searches. However, it’s also helpful to go beyond these and scour through smaller, more subject-specific databases.

Here are some databases that you can use to find studies to inform your systematic review.

Health Sciences Databases

  • Biosis Citation Index – journals, books, reviews, conferences, and patents on life and biomedical sciences
  • ChiroACCESS – peer-review articles on alternative medicine, such as chiropractic, homeopathy, osteopathy, etc.
  • CINAHL Complete – abstracts, dissertations, publications, conferences, evidence-based care sheets, and instrument descriptions in nursing and allied health
  • Cochrane Central Register of Controlled Trials (CENTRAL) – reports on randomized and quasi-randomized controlled trials
  • Embase – bibliography reports in biomedicine and pharmaceuticals
  • Joanne Briggs Institute EBP Database – information sheets, systematic reviews, electronic journals, conference papers, and consumer healthcare information of evidence-based research
  • PubMed – life science journals, online books, and full-text content in medicine and related fields

Education Databases

  • Education Research Complete – journals and full texts of books, monographs, and conference papers in education and educational specialties
  • ERIC – library of education research and information, including grey literature
  • Web of Science – scholarly citations and abstracts in the sciences, arts, and humanities

Social Studies Databases

  • Criminal Justice Abstracts – full texts in criminal justice
  • EconLit – electronic database of economic literature
  • ProQuest – journals, magazines, and theses in business, management, and economics
  • PsycINFO – peer-review journals, articles, books, dissertations, etc. in psychology
  • Scopus – materials and citation analyses in humanities and sciences
  • Sociological Abstracts – literature on sociology and behavioral sciences

How to Search for Studies for a Systematic Review

Searching, appraising, and synthesizing studies is the heart of a systematic review, and these steps take up the bulk of the research methodology. It can be time-consuming and tedious. However, the process can be made easier with a literature review software like DistillerSR, which automates key steps to help you gather enough studies from multiple databases and yield accurate findings faster and more efficiently.

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