The protocol is the detailed plan for your systematic review. It explains your rationale and methodology, and should be developed right at the beginning. You can refine it as you go on.
Publishing your protocol in either a register or a journal will help avoid duplication - you don’t want other researchers doing the same review as you. It also “…reduces the impact of review authors’ biases, promotes transparency of methods and processes ... and allows peer review of the planned methods.” Cochrane Handbook, Part 2, Chapter 1.5
If you need more convincing, here are some further arguments for a registered protocol:
It’s also a good idea to check through protocol registers to see if another group of researchers has already registered a review on your topic. (See the "where" box below).
This is a guideline to help authors prepare protocols for planned systematic reviews and meta-analyses that provides them with a minimum set of items to be included in the protocol. Check the PRISMA-P checklist and the PRISMA-P statement.
For more information and a good explanation, read this article:
Shamseer, L., Moher, D., Clarke, M., Ghersi, D., Liberati, A., Petticrew, M., . . . Prisma- P. Group. (2015). Preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015: elaboration and explanation. BMJ, 350, g7647. doi:10.1136/bmj.g7647
Typically, your systematic review protocol should include:
Once you register your protocol on PROSPERO, it will be available on open access through their database. Your protocol will have a unique registration number which can be cited in publications and reports to provide the link between your planned and completed review.
Find out more here: Guidance notes for registering a systematic review protocol with PROSPERO (University of York Centre for Reviews and Dissemination).
Many journals will publish protocols - check the journals in your field. Authors’ instructions will often have guidelines for the level of detail expected for the protocols they publish.