What is the minimum number of included studies in a systematic review?
This question has been asked with different wording:
- How many studies do I need for my systematic review? Zero.
- How many studies are enough for a meta-analysis? 2 or more (depends, read on).
- Can I publish a systematic review with no studies? Yes.
These are interesting questions that my students and colleagues ask their supervisors, statisticians, librarians or information specialist. The reason behind the question or the purpose of the systematic review would change the answer from the right and easy answer, which is zero, to more complicated answers such as 2, 5, 10, or as many as possible.
There is no minimum or a maximum number of studies for inclusion or exclusion in a systematic review. Many researchers — by mistake — scream Eureka Archimedes-style when they found nothing for their systematic reviews.
Systematic Review could be done for many purposes and the number of the included study or studies would be dependent on the purpose.
Purpose 1: Coursework, assignment, master's or PhD thesis dissertation
Yeah, you wish! Your supervisor should be an angel to let you get on with no included studies! Even if you have an excellent question that has no answer in the literature, the purpose of conducting a systematic review as part of your education is to learn! To learn a systematic review, you should learn (don't think about doing everything perfectly):
- To manage a team and a project
- To develop a review protocol (proposal)
- To design and run searches with assistance from a librarian
- To select the relevant studies among the search results
- To learn the methods to find the full text of the records
- To critically appraise the quality of the studies using relevant tools
- To pull the data and information out of studies and pool them into a table or figure for data analysis/synthesis
- To write a report