Inclusion and Exclusion Criteria for Reviews

Inclusion and exclusion criteria are the predefined rules that determine which studies are eligible for your systematic review. They are the most consequential methodological decision you make after formulating your research question, because they directly control what evidence enters your review and therefore what conclusions you can draw. Poorly defined criteria lead to inconsistent screening, missed relevant studies, inclusion of irrelevant studies, and reviewer disagreements that delay the entire review process. Once your rules are set, screen records against your eligibility criteria with structured exclusion reasons that tally automatically.

Eligibility criteria must be established during protocol development, before the search begins, and documented in your PROSPERO registration and protocol. Changing criteria after screening starts introduces selection bias and violates the transparency principles that distinguish systematic reviews from narrative literature reviews. This guide covers how to develop clear, operational criteria based on your research question, apply them consistently across reviewers, handle borderline cases, and document everything for our guide to prisma 2020 compliance.

What Are Inclusion and Exclusion Criteria?

Exclusion criteria specify characteristics that disqualify a study, even if it otherwise meets inclusion criteria. Exclusion criteria address factors like publication type, language, or specific methodological features that make a study unsuitable.

Together, these criteria define your review's eligibility framework. Every study identified by your search is assessed against this framework, first at the title-abstract screening stage and then again at full-text review. The process must be conducted by at least two independent reviewers to minimize individual bias.

The Relationship Between PICO and Eligibility Criteria

Your eligibility criteria are directly derived from your research question, which should be structured using the PICO framework (or PCC for scoping reviews). Each PICO element translates into one or more eligibility criteria.

How to Write Clear Inclusion Criteria

Effective inclusion criteria are specific, measurable, and unambiguous. Every reviewer reading your criteria should reach the same decision about the same study. Vague criteria are the single biggest source of screening disagreements and reviewer frustration.

Population Criteria

Define your target population with enough specificity to exclude irrelevant studies while remaining broad enough to capture all relevant evidence.

Too vague: "Studies involving patients with diabetes" Clear: "Adults aged 18 years or older with a clinical diagnosis of type 2 diabetes mellitus, diagnosed using WHO or ADA criteria. Studies including mixed populations are eligible if results for type 2 diabetes are reported separately."

Key population considerations:

• Age range: Specify exact boundaries (18+, 0-17, 65+)
• Diagnosis: Reference specific diagnostic criteria or classification systems
• Setting: Inpatient, outpatient, community, primary care
• Geography: Worldwide or limited to specific regions (justify any restriction)
• Mixed populations: State whether studies with mixed populations are included and under what conditions

Intervention Criteria

Define the intervention precisely enough that reviewers can consistently identify it across different study descriptions.

Too vague: "Exercise interventions" Clear: "Structured exercise programs involving at least 150 minutes per week of moderate-intensity aerobic activity, delivered for a minimum of 8 weeks, with documented supervision or adherence monitoring. Home-based and facility-based programs are both eligible."

Key intervention considerations:

• Type: What specific intervention or exposure
• Dose/intensity: Minimum thresholds for inclusion
• Duration: Minimum intervention length
• Delivery: Who delivers it, in what setting
• Fidelity: Any requirements for intervention adherence or protocol compliance

Comparison Criteria

Specify what comparators are acceptable, or state that any comparator (including no comparator) is eligible.

Examples:

• "Any active comparator, placebo, usual care, or waitlist control"
• "Usual care or no intervention only (active comparator studies excluded)"
• "No comparator required (single-arm studies eligible)"

Outcome Criteria

Define which outcomes must be reported for a study to be included. Decide whether you require specific outcomes or will include studies reporting any relevant outcome.

Strict approach: "Studies must report at least one of the following primary outcomes: all-cause mortality, cardiovascular events, or HbA1c change from baseline" Flexible approach: "Studies reporting any patient-relevant outcome related to diabetes management"

Study Design Criteria

Specify which study designs are eligible. This is one of the most impactful criteria because it determines the volume and type of evidence in your review.

How to Write Clear Exclusion Criteria

Exclusion criteria should address factors not covered by your inclusion criteria. Common exclusion criteria include:

• Publication type: Exclude conference abstracts only, editorials, commentaries, letters, narrative reviews (specify which)
• Language: State language restrictions if any (and justify them)
• Date range: Specify if studies before a certain date are excluded (e.g., "Published before January 2000")
• Duplicate populations: "If multiple publications report the same study population, only the most comprehensive or most recent publication will be included"
• Specific methodological features: "Studies with fewer than 10 participants per arm" or "Studies without a control group"
• Full text availability: "Studies for which the full text could not be obtained after contacting authors"

Important: Exclusion criteria should not simply restate the inverse of inclusion criteria. If your inclusion criterion is "adults aged 18+," you do not need a separate exclusion criterion for "studies of children." Only add exclusion criteria that address additional factors beyond what inclusion criteria already define.

Need expert support developing your eligibility criteria and managing the screening process? Our methodologists build precise criteria, validate them against pilot screening samples, and conduct dual-reviewer screening for your entire review. request a consultation for your study to discuss your project, or see our systematic review screening support.

Pilot Testing Your Criteria

Before beginning full screening, pilot test your criteria on a sample of 50 to 100 records with all reviewers. This is one of the most important steps in the systematic review process and is frequently skipped, leading to problems that cost weeks to resolve later.

How to Pilot Test

1. Select a random sample of 50 to 100 title-abstract records from your search results
2. Have all reviewers independently screen the sample using your eligibility criteria
3. Compare decisions and calculate inter-rater agreement (Cohen's kappa or percent agreement)
4. Discuss all disagreements and identify which criteria caused confusion
5. Revise criteria wording to resolve ambiguities
6. Repeat with a new sample if agreement was below 80 percent
7. Document the final, refined criteria in your protocol

What Good Agreement Looks Like

• Kappa above 0.80: Excellent agreement, proceed with full screening
• Kappa 0.60 to 0.80: Substantial agreement, minor revisions may help
• Kappa below 0.60: Moderate or poor agreement, criteria need significant revision

Our Cohen kappa statistics tool can help you assess inter-rater agreement during pilot testing.

Applying Criteria During Screening

Title-Abstract Screening

At this stage, reviewers assess each record based on the information available in the title and abstract. The general rule is include if uncertain. If the title and abstract do not provide enough information to confidently exclude a study, it should advance to full-text review.

Use systematic review software like Covidence or Rayyan for efficient dual-reviewer screening with conflict resolution. For guidance on building your learn about search strategy that feeds into screening, see our dedicated guide.

Full-Text Screening

At full-text review, apply your criteria more rigorously using the complete study report. For each excluded study, record the primary reason for exclusion using predefined categories that correspond to your eligibility criteria. These reasons populate the "Excluded studies" section of your PRISMA flow diagram.

Common exclusion reason categories:

1. Wrong population
2. Wrong intervention/exposure
3. Wrong comparator
4. Wrong outcome
5. Wrong study design
6. Duplicate publication
7. Full text unavailable
8. Wrong publication type (e.g., conference abstract only)

Handling Borderline Cases

Borderline studies are inevitable. Establish a process for handling them before screening begins:

• Decision rules: Create written rules for predictable borderline scenarios (e.g., "If a study includes both adults and adolescents, include if at least 80 percent of participants are adults")
• Third reviewer: Designate a third reviewer or senior methodologist to resolve persistent disagreements
• Sensitivity analysis: Consider including borderline studies in a sensitivity analysis to assess whether their inclusion or exclusion changes the review's conclusions

Documenting Criteria for PRISMA Compliance

learn about prisma 2020 requires transparent reporting of eligibility criteria. Specifically:

• Item 5 (Eligibility criteria): "Specify the inclusion and exclusion criteria for the review and how studies were grouped for the syntheses"
• Item 16 (Selection process): "Describe the results of the search and study selection process, including the number of records identified, screened, assessed for eligibility, and included"

Present your criteria as a structured table or clearly formatted list in the Methods section. Include specific definitions, thresholds, and examples for any criteria that could be interpreted differently by different readers.

Example Criteria Table

Common Mistakes to Avoid

1. Criteria Too Broad

Overly inclusive criteria generate thousands of irrelevant records that must still be screened, wasting months of reviewer time. If your search returns 15,000 records and you include only 30 studies, your criteria may need tightening.

2. Criteria Too Narrow

Excessively restrictive criteria risk excluding relevant evidence and producing a review with too few studies to draw meaningful conclusions. If your search returns only 200 records after applying eligibility criteria, consider whether your restrictions are necessary.

3. Vague or Subjective Language

Criteria containing words like "relevant," "significant," "appropriate," or "high quality" without operational definitions will produce inconsistent screening decisions. Every criterion should be concrete enough that two independent reviewers reach the same conclusion.

4. Adding Criteria After Screening Begins

Post-hoc criteria changes introduce selection bias. If you realize during screening that a criterion needs modification, document the change as a protocol amendment with justification and apply it consistently to all records, including those already screened.

5. Not Specifying Date or Language Restrictions

If you have no date restriction, state "No date restriction applied." If you have no language restriction, state "No language restriction applied." Omitting these statements leaves reviewers and readers uncertain about your search scope.

6. Inconsistent Application

Ensure all reviewers apply criteria identically. Calibration through pilot testing, written decision rules, and regular team discussions prevents drift over the course of a lengthy screening process.

Applying criteria consistently is easier with dual screening. Read about the role of a second reviewer in screening to reduce selection bias.