Can You Change Your Inclusion Criteria After Screening Has Started?

What If Screening Reveals That Your Original Criteria Do Not Quite Work?

You carefully define your inclusion and exclusion criteria before screening. Then you begin looking at the actual literature and discover something you did not anticipate.

Perhaps studies describe your population differently from the terminology you used in the protocol. Maybe most interventions combine the component you wanted to study with another component. A previously unconsidered study design turns out to be common. Or your definition of “higher education” becomes unexpectedly difficult to apply to vocational colleges, polytechnics, and other postsecondary institutions.

At that point, changing the criteria can feel methodologically suspicious. After all, one reason for defining eligibility criteria in advance is to prevent researchers from adjusting the rules after seeing the evidence.

But prespecification does not mean pretending that unforeseen methodological problems never occur. Systematic review guidance recognizes that changes can sometimes be necessary. The important questions are why the criterion needs to change, whether the change remains faithful to the review objective, whether knowledge of study findings influenced the decision, and how transparently the amendment is handled.

Yes, but the Reason for the Change Matters

Prespecification Protects Against Bias, but It Does Not Eliminate the Need for Judgment

Why Eligibility Criteria Are Defined Before Screening

Systematic reviews are intended to identify evidence using methods that are explicit enough to reduce opportunities for selective decision-making. Prespecified eligibility criteria are central to that purpose.

Cochrane describes following prespecified eligibility criteria as a fundamental attribute of a systematic review. The criteria are ordinarily developed from the review question and establish which populations, interventions or exposures, comparators, study designs, and other characteristics define eligible evidence.

Prespecification matters because researchers eventually learn things about the available studies. Once you know which papers report large effects, which support your hypothesis, or which would change the synthesis, eligibility decisions can become vulnerable to those findings.

A protocol therefore does more than organize the project. It creates a record of important methodological decisions before their consequences are fully known.

But Protocols Are Plans, Not Predictions of Every Study You Will Encounter

No protocol can anticipate every feature of a body of literature.

Cochrane explicitly recognizes that unforeseen issues may arise and requires changes to eligibility criteria to be justified rather than prohibiting them altogether. Its guidance also acknowledges that review questions may sometimes require refinement as researchers develop a fuller understanding of the evidence.

The critical boundary is that such changes should not be made because researchers have seen how the studies turn out. Cochrane cautions that changes based on study findings or synthesis results can introduce bias.

This produces a more nuanced rule than “never change the protocol”:

The first may be defensible. The second creates a much more serious risk of bias.

Clarifying a Criterion Is Not Always the Same as Changing It

Before describing something as a protocol amendment, determine what actually happened.

Suppose your eligibility criteria say that studies must involve “higher-education students.” During pilot screening, reviewers disagree about whether students enrolled in polytechnics qualify.

If the intended scope always encompassed recognized postsecondary institutions and the problem is simply that the wording was insufficiently operationalized, defining the term more precisely may be a clarification.

Now suppose the protocol explicitly says “students enrolled in bachelor's or postgraduate degree programs,” but you decide after screening begins that vocational students should also qualify. That expands the eligible population and is a substantive change.

The distinction between clarification and amendment can itself require judgment. A useful test is whether the decision changes which kinds of studies can enter the review. If it does, treating the change as substantive is usually the more transparent approach.

Screening Can Legitimately Reveal Problems You Could Not See Earlier

Pilot screening exists partly because criteria that look clear in a protocol can behave differently when applied to actual records.

You may discover that:

• authors use terminology differently from what you anticipated;
• populations commonly span categories you had treated as separate;
• an intervention almost always appears as part of a multicomponent program;
• study designs are reported using inconsistent labels;
• the boundary between two settings is difficult to operationalize;
• an apparently straightforward publication-date rule produces unintended consequences; or
• a criterion does not actually distinguish the evidence relevant to the review question.

These problems do not automatically justify changing the criteria. They do justify examining whether the original rule remains conceptually and methodologically defensible.

This is one reason consistent application of inclusion and exclusion criteria requires more than mechanically following ambiguous wording. The objective is consistent eligibility decisions that remain aligned with the review question.

The Most Important Test Is Whether the Change Depends on the Findings

Consider two very different reasons for expanding an eligible population.

Reason A: During early screening, you discover that studies routinely combine first- and second-year university students, although your original criterion inadvertently specified first-year students only. Your review question was intended to concern beginning university students more broadly, and you recognize that the criterion does not represent the intended construct well.

Reason B: You discover a second-year student study reporting an unusually large effect that would strengthen your argument, so you decide that second-year students should count after all.

The amendment may look identical on paper. Its methodological meaning is not.

Cochrane states that protocol changes must not be made on the basis of the findings of the studies or the synthesis. This is because knowledge of results can influence methodological choices in ways that favor particular conclusions.

Do Not Change the Criteria Just to Get a Better Number of Studies

Another tempting reason for changing eligibility is the size of the evidence base.

Your criteria produce only four eligible studies, so you broaden them until you have twenty. Or your search produces hundreds of potentially eligible studies, so you add restrictions to make the workload manageable.

Neither situation automatically justifies an amendment.

A small evidence base may be an important finding about the state of research. A large evidence base may require additional resources or a more carefully scoped question, but arbitrary post hoc restrictions can alter what the review is answering.

Sometimes the scope genuinely is too broad or too narrow and revision is appropriate. The justification, however, should concern the coherence, feasibility, or scientific purpose of the review rather than reaching a preferred number of included studies.

Changing Eligibility Can Mean You Need to Revisit Earlier Screening

A criterion change does not apply only from the moment you make it.

Suppose you originally excluded vocational-college students but later expand the population to include all tertiary education. Records excluded earlier because they involved vocational students may now be eligible.

Those decisions need to be revisited.

Otherwise, studies encountered before the amendment and studies encountered afterward may effectively be screened against different protocols.

Expanding the Criteria May Require a New Search

This consequence is particularly easy to overlook.

Suppose your original review includes only university students, so your search strategy was designed around university and higher-education terminology. Halfway through screening, you expand eligibility to secondary-school students.

Simply rescreening your existing results may not be enough. The original search may never have been designed to retrieve school-based studies comprehensively.

Cochrane's guidance on updated reviews makes the same methodological point: when important changes are made to the review question or eligibility criteria, parts of the search may need to be rerun back to the earliest records to ensure that studies relevant under the revised scope were not missed.

Whenever eligibility expands, ask:

Would my original search have identified the newly eligible evidence?

If the answer is uncertain, the search strategy may need revision and rerunning.

Narrowing the Criteria Can Also Require Reassessment

If the amendment makes eligibility narrower, studies already included under the earlier rule may no longer qualify.

That creates an obvious methodological consequence. You cannot normally apply the stricter rule only to studies screened after the amendment while allowing earlier studies to remain because they have already been processed.

Reapply the revised criterion to the relevant earlier records and document the resulting changes.

If the proposed narrowing occurs because certain studies complicate the synthesis or weaken the apparent effect, that should raise particular concern. Methodological inconvenience and undesirable results are not interchangeable with scientific justification.

An Important Study Is Not, by Itself, a Reason to Rewrite the Rules

One of the hardest cases occurs when you discover an influential study that does not meet the original criteria.

The study may be foundational. Everyone in the field cites it. Excluding it can make the review feel incomplete.

But prominence does not establish eligibility. If the study clearly falls outside a valid criterion, it may still be appropriate to discuss it as theoretical, historical, methodological, or contextual literature without counting it among the formally included studies.

The problem of handling an important study that does not meet the original criteria should therefore be separated from the question of whether the criteria themselves are defective.

Protocol, Registration, and Review Report Should Tell the Same Methodological Story

When a substantive amendment occurs, transparency requires more than remembering it yourself.

Cochrane requires methodological changes from the protocol to be transparently declared. PRISMA 2020 likewise asks authors to indicate whether a review protocol exists and to describe and explain amendments to information provided at registration or in the protocol.

Depending on the review and the platform used, this may involve:

• dating the amendment in the project records;
• recording the original criterion;
• recording the revised criterion;
• explaining why the change was necessary;
• updating the protocol or registration where the system permits or requires it;
• describing the amendment in the completed review; and
• explaining any resulting changes to searching, screening, or analysis.

The objective is not bureaucratic perfection. It is to allow readers to distinguish decisions planned before the evidence was examined from decisions made later in response to unforeseen circumstances.

Changing the Review Question Is a Larger Decision Than Clarifying Eligibility

Sometimes what looks like an eligibility problem is actually evidence that the research question itself has changed.

If your review originally asks about university students and you decide to include all learners from primary school through postgraduate education, you may no longer be refining a population criterion. You may be asking a materially different question.

Cochrane recognizes that review questions can sometimes be modified as understanding of the evidence develops, but emphasizes the need to guard against data-driven questions and to document changes to the protocol.

When a proposed eligibility amendment substantially changes the population, intervention, exposure, comparison, outcomes, or eligible study designs, ask whether the project is still the same review. In some circumstances, revising the protocol substantially or beginning a new review may be more methodologically coherent than stretching the original question beyond recognition.

Different Review Types Carry Different Expectations

The concern about protocol amendments is strongest for systematic and similarly structured evidence syntheses because prespecification is part of their methodological architecture.

JBI guidance likewise expects systematic review protocols to state explicit, unambiguous inclusion criteria, and its reporting guidance calls for protocol amendments to be reported. Scoping reviews also normally establish eligibility criteria prospectively, although their broader exploratory purposes can lead to different decisions about how criteria are formulated and refined.

A traditional narrative literature review may not operate from a registered protocol or formal eligibility framework at all. In that setting, the issue is less about violating a protocol and more about maintaining intellectual transparency. If the scope evolves substantially while reading, make sure the final review accurately represents what literature was considered and does not imply a systematic selection procedure that was never actually used.

A Criterion Changes Halfway Through Screening: What Happens Next?

This amendment is not methodologically invisible. It changes the eligible population. But it can still be defensible if the reasoning follows the review objective rather than the findings and the consequences are handled consistently.

Now change one detail. Imagine that the polytechnic studies all report unusually positive effects, and the team broadens eligibility specifically because including them makes the synthesis more favorable. The wording of the amendment could be identical, yet its methodological credibility would be very different.

Common Mistakes When Changing Eligibility Criteria

Change the Criteria Only When You Can Defend the Change Without Referring to the Results

When a criterion begins to look problematic during screening, do not immediately change it. First diagnose what kind of problem you actually have.

Before finalizing the change, write down the rationale without mentioning any study's result. If the justification remains coherent, that is reassuring. If the explanation keeps returning to statements such as “otherwise we lose this important positive study,” you have identified a different problem.

Finally, preserve the history of the decision. The final review should not make a midstream amendment look as though it had been planned from the beginning.

Before Changing an Inclusion or Exclusion Criterion

Questions About Changing Eligibility Criteria During a Review

You Can Change the Criteria, but You Cannot Pretend You Never Did

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