01 · The Question
Does Every Literature Review Need a Written Protocol Before You Begin?
You have a review question and are ready to search the literature. Before opening Scopus, Web of Science, PubMed, or another database, do you need to write a protocol?
The answer depends on what kind of literature review you are conducting.
A literature review used to establish the background for an empirical paper does not ordinarily require a formal systematic-review protocol. A narrative review may use a flexible planning document without following a standardized protocol framework. A systematic or scoping review, however, is methodologically different because explicit decisions about eligibility, searching, selection, appraisal, and synthesis form part of the research design itself.
For those reviews, deciding the methods before the results of the search begin influencing your choices is an important safeguard against avoidable bias and methodological drift.
03 · What You Need to Know
What a Review Protocol Does and When You Need One
A Protocol Is the Research Plan for the Review
A review protocol is a prospective description of the rationale, objectives, and methods that will guide an evidence synthesis. It establishes important decisions before the review has progressed far enough for knowledge of particular studies or findings to influence those decisions.
For a systematic review, the protocol may specify the review question, eligibility criteria, information sources, search strategy, study-selection procedures, data items, risk-of-bias assessment, synthesis methods, planned subgroup or sensitivity analyses, and other methodological decisions relevant to the review.
The protocol therefore serves much the same intellectual purpose as a research protocol for a primary empirical study. The evidence already exists, but the researchers still make consequential methodological decisions about how it will be identified, selected, analyzed, and interpreted.
Not Every Document Containing a Literature Review Needs a Review Protocol
The term literature review covers several different activities. As discussed in the distinction between a literature review and a review article, reviewing prior scholarship may be one component of a larger empirical project or may constitute the research project itself.
If you are reviewing literature to write the background section of an empirical paper, you usually do not need to create a formal evidence-synthesis protocol. You still need a thoughtful search and responsible engagement with the literature, but the purpose is not necessarily to establish a systematically selected evidence base from which review-level conclusions will be drawn.
The case changes when the review itself is the study and its credibility depends substantially on how sources are identified and synthesized.
Systematic Reviews Should Be Planned Prospectively
Cochrane states that all systematic reviews should have a protocol that describes the review rationale, hypothesis, and planned methods. Prospective planning reduces the risk that methods will be changed according to knowledge of individual study findings.
For example, imagine discovering midway through screening that excluding a particular study design removes several studies with results that contradict your expectation. If the eligibility criteria were defined prospectively, readers can distinguish a planned methodological decision from a post hoc change potentially influenced by the emerging evidence.
A protocol cannot eliminate reviewer judgment or bias. It makes consequential decisions more visible and creates a reference against which later changes can be explained.
Scoping Reviews Also Benefit From an A Priori Protocol
A scoping review is broader than many systematic reviews, but broader does not mean methodologically unplanned. JBI states that an a priori protocol should be developed before conducting a scoping review and that the protocol should define the review objective, question, inclusion criteria, and methods.
This is particularly useful because scoping reviews can involve broad eligibility criteria and large, heterogeneous evidence bases. Without prospective boundaries, researchers can easily change the scope repeatedly as new literature appears.
If you are still deciding which approach fits the question, resolve the distinction between systematic and scoping review before writing a protocol designed for the wrong review methodology.
Narrative Reviews Are More Variable
Narrative reviews generally do not have one universally required protocol framework. Their objectives may be conceptual, theoretical, historical, educational, or interpretive, and their methods often permit greater flexibility than systematic evidence synthesis.
That does not mean planning is unnecessary. A narrative reviewer can still define the review purpose, boundaries, information sources, broad search approach, principles for source selection, analytical organization, and procedures for addressing conflicting literature.
Whether that planning document should be called a formal protocol depends on the methodology and disciplinary context. The key point is not to impose systematic-review conventions on every form of scholarship merely because protocols are valuable in systematic reviews.
| Review Situation |
Formal Protocol? |
Practical Interpretation |
| Literature review within an empirical article |
Usually not required as a separate review protocol |
Use a defensible literature-search and synthesis process appropriate to the article's purpose |
| Thesis or dissertation background chapter |
Depends on program and methodology |
A search plan may be valuable, but a formal evidence-synthesis protocol is not automatically required |
| Narrative review article |
Not universally required |
Prospective planning can improve transparency and reduce arbitrary source selection |
| Systematic review |
Generally expected |
Develop the protocol prospectively according to the relevant review methodology |
| Scoping review |
Expected under established methodologies such as JBI |
Define the objective, question, eligibility, search, selection, charting, and synthesis approach prospectively |
| Rapid review |
Still important |
Specify the planned methodological restrictions and efficiencies rather than deciding them opportunistically |
| Umbrella review |
Expected under established methodologies |
Plan review-level eligibility, appraisal, overlap management, extraction, and synthesis in advance |
A Protocol Is More Than a Search Strategy
Researchers sometimes prepare a list of keywords and databases and call it a protocol. Searching is important, but it is only one part of the review design.
A useful protocol explains what question the review addresses, which evidence is eligible, how records will be screened, what information will be extracted, how included evidence will be appraised where applicable, and how findings will be synthesized. It may also specify procedures for resolving disagreements, handling multiple reports of the same study, managing missing information, and conducting planned additional analyses.
The exact contents depend on the review methodology. Do not add methodological sections merely because another protocol contains them if they do not apply to your review.
PRISMA-P Provides Reporting Guidance for Systematic Review Protocols
PRISMA-P was developed to improve reporting of protocols for systematic reviews and meta-analyses. Its checklist addresses administrative information, introduction, and methods, including eligibility criteria, information sources, search strategy, study records, data items, outcomes, risk of bias, and synthesis.
PRISMA-P is a reporting guideline. It helps authors report a protocol completely, but it is not a substitute for methodological guidance on how the systematic review should be designed.
The methodology should come from guidance appropriate to the type of review being conducted, such as Cochrane or JBI where applicable.
A Protocol Can Change
Writing a protocol does not require pretending that every methodological problem can be predicted before the review begins. Evidence syntheses often encounter unexpected circumstances.
Perhaps an outcome is reported differently from what you anticipated. A planned database becomes inaccessible. A supposedly manageable search retrieves tens of thousands of records. A proposed statistical synthesis proves inappropriate after the included studies are examined.
Changes can be legitimate. The important practice is to document substantive amendments, explain why they were necessary, and distinguish them from the original plan. Cochrane emphasizes reporting and justifying protocol changes rather than silently rewriting the plan after the results are known.
Protocol amendment
A documented change to the planned review methods, with the timing and rationale made transparent.
Undisclosed post hoc decision
A methodological change made after the review has begun without transparently explaining that it differed from the original plan.
Protocol, Registration, and Protocol Publication Are Different Things
These terms are often treated as interchangeable, but they refer to different actions.
You can write a detailed protocol and keep it within the research team. You can register information about the planned review in an appropriate registry or repository. You may also publish the full protocol as a journal article or another publicly accessible document.
Public registration can increase transparency, establish a time-stamped record, and help other researchers discover ongoing reviews. Protocol publication can provide substantially more methodological detail and subject the protocol to editorial or peer review where applicable.
Neither should be confused with simply having a protocol. The separate question of whether a systematic or scoping review should be preregistered therefore deserves its own decision.
A Protocol Does Not Make a Review Rigorous by Itself
A poorly designed method does not become appropriate because it was written down before the review began. The protocol needs a defensible question, suitable eligibility criteria, appropriate search methods, valid appraisal and synthesis procedures, and enough methodological detail to guide the work.
Similarly, adherence to a protocol is not always desirable when an unforeseen problem makes the planned method inappropriate. Transparency about a justified change is preferable to rigidly following a flawed plan simply to claim perfect protocol adherence.
Watch Out
Do not write the protocol after screening or analysis and present it as though it were prospective. A protocol's methodological value comes partly from documenting intentions before the evidence has had substantial opportunity to influence them. If planning occurred retrospectively, describe it accurately.
