All systematic reviews require a written protocol that specifies the plan which the review will follow to identify, appraise and collate evidence. The protocol makes the review process transparent, helps to avoid or minimise bias, avoids duplication, is developed by reviewers with expertise in the area and it allows peer review.
Components of a protocol
The CRD (Centre for Reviews and Dissemination) has useful guidance on the review protocol.
CRD Guidance 1.2 - The Review Protocol
Scoping searches
As part of your preparation of your protocol you may want to do some simple scoping searches. This is a quick search on a key database to find out the level of literature that has already been written on the subject. It also might help you define the limits of your search. You can also use these searches to start harvesting search terms to use later on in your search strategy.
You could use text mining to help you come up with some search terms. PubMed PubReMiner can be used to run a simple search on PubMed and will give you commonly used free text and MeSH headings for your search.
You should also check that your review has not already been done or is not currently in progress. You can check the following for registered protocols (in the health-care field):
Cochrane review protocols can be found in the Cochrane Library (available from the Library's databases list).
Registering your protocol
You should register your final protocol on PROSPERO to publicise that you are doing the research (if your review is health-care related).
Next step: Creating a search strategy
Inclusion and exclusion criteria need to be defined in advance to decide which studies should be included in the systematic review analysis. These need to be included in the protocol.
"The review protocol should provide explicit, unambiguous, inclusion criteria for the review."
The JBI Manual for Evidence Synthesis, 2020
Inclusion criteria based on study characteristics - types of participants (ages etc), settings, types of interventions, comparators, types and measurements of outcomes, types of studies
Inclusion criteria based on publication characteristics - publication date, language of publication, type of publication
"The population, intervention and comparison components of the question, with the additional specification of types of study that will be included, form the basis of the pre-specified eligibility criteria for the review."
Chapter 3 of the Cochrane Handbook has lots of advice on selecting your inclusion or eligibility criteria.