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.
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.
| PICO Element | Becomes | Example |
|---|---|---|
| Population | Who is included | Adults over 18 with type 2 diabetes |
| Intervention | What treatment/exposure | Cognitive behavioral therapy delivered by trained therapists |
| Comparison | What alternative | Usual care, waitlist control, or active comparator |
| Outcome | What is measured | HbA1c levels, self-reported quality of life |
| Study design | What designs qualify | Randomized controlled trials, quasi-experimental studies |
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.
| Decision | Effect |
|---|---|
| randomized controlled trials only | Fewer studies, stronger causal inference, meta-analysis straightforward |
| randomized controlled trials + quasi-experimental | More studies, mixed quality, separate analysis needed |
| All quantitative designs | Maximum evidence, complex quality assessment, multiple RoB tools needed |
| Include qualitative | Requires qualitative synthesis methods (JBI meta-aggregation or thematic synthesis) |
