03 · What You Need to Know
How to Determine Whether Research Eligibility Criteria Are Unnecessarily Restrictive
What Are Inclusion and Exclusion Criteria Supposed to Accomplish?
Inclusion criteria identify the characteristics required for participation. These may concern the condition being studied, relevant exposure, age range, research setting, or other characteristics needed to answer the research question.
Exclusion criteria identify circumstances that make participation inappropriate despite an individual otherwise meeting the inclusion requirements. Examples may include a contraindication to a study intervention, a condition that creates unacceptable additional risk, or a circumstance that makes the intended outcome impossible to assess reliably.
The distinction is operational rather than absolute. Some protocols express eligibility using inclusion requirements, while others state corresponding exclusions. What matters is the substantive justification and effect of the criteria.
Well-designed eligibility criteria should define a population suitable for answering the research question while avoiding restrictions that are unnecessarily broad.
What Makes an Eligibility Restriction Unnecessary?
A restriction may be unnecessary when the research objective can be achieved without it, the associated risk can be managed through appropriate safeguards, or the restriction is based on a characteristic that does not meaningfully determine whether someone can participate.
For example, excluding every person who uses assistive technology from a questionnaire study may be difficult to justify if the instrument can be administered accessibly without changing the construct being measured.
Similarly, excluding all older adults from an observational study solely because the researchers anticipate slower recruitment may simplify administration but does not establish a scientific rationale.
Researchers should distinguish restrictions that define the population of interest from those introduced to make recruitment or data collection more convenient.
Scientifically necessary restriction
Required to define the research population, maintain an interpretable comparison, or answer the specified research question.
Safety-based restriction
Supported by a study-specific assessment showing that participation would create an unacceptable risk or require safeguards that cannot adequately manage it.
Potentially unnecessary restriction
Based primarily on convenience, assumptions, historical practice, or characteristics that do not independently justify exclusion.
Should Older Adults Be Excluded Because of Their Age?
Chronological age alone is not necessarily a sufficient reason to exclude older adults. The relevant considerations may instead involve physiological characteristics, intervention risks, functional requirements, or the specific population the study intends to investigate.
For example, a clinical trial involving an intervention intended for older patients may have limited applicability if it excludes people above an arbitrary age threshold. Conversely, a study explicitly investigating a developmental phenomenon among adolescents may appropriately restrict participation by age because age is integral to the research question.
Age-related changes in organ function, medication use, or comorbidities may be clinically relevant in certain intervention studies. Researchers should assess those factors directly where feasible rather than assuming that everyone above a particular age has the same risk profile.
The United States National Institutes of Health (NIH) requires inclusion across the lifespan in NIH-supported human-subjects research unless scientific or ethical reasons justify exclusion. Its policy specifically requires a rationale for age-related restrictions. This is a funder-specific requirement, not a universal rule governing all research worldwide.
Where age limits are proposed, researchers should explain why the chosen threshold is necessary and whether the intended population includes people beyond it.
When Does Disability Become an Unjustified Basis for Exclusion?
Disability is not a single clinical or functional condition. People with disabilities have varied capabilities, support needs, communication preferences, and circumstances. A blanket exclusion may therefore substitute a broad label for the actual requirements of participation.
Suppose a researcher excludes people with visual impairments from an interview study because recruitment materials are distributed as scanned documents. The obstacle may be the format of the materials rather than an inability to contribute meaningful data.
In another study, a particular functional requirement may be directly relevant. For instance, an investigation specifically measuring unaided visual perception may require participants to perform defined visual tasks. The scientific justification should concern the measurement requirement rather than a generalized judgment about disability.
Researchers should consider accessible research materials and appropriate accommodations before concluding that disability-related participation barriers cannot be addressed.
Accommodations may include alternative document formats, accessible interfaces, communication support, modified scheduling, or changes to the physical research environment. Whether a particular accommodation is appropriate depends on participant needs, the research procedures, and the validity of the resulting measurements.
Can Researchers Exclude People Who Have Difficulty Providing Informed Consent?
Researchers must distinguish disability, communication difficulty, and impaired decision-making capacity. These are not interchangeable.
A participant who communicates through assistive technology may be fully capable of making an informed decision. Someone with limited literacy may understand the research when information is explained appropriately. Neither circumstance automatically establishes an inability to consent.
For adults who lack decision-making capacity for a particular research decision, additional ethical and legal safeguards may be necessary. Depending on the applicable jurisdiction, study characteristics, and ethics approval, participation may involve a legally authorized representative, appropriate assent procedures, or other protections.
The Council for International Organizations of Medical Sciences (CIOMS), in collaboration with the World Health Organization (WHO), addresses research involving adults incapable of giving informed consent in Guideline 16 of its 2016 International Ethical Guidelines.
Researchers should not assume that surrogate permission is automatically available or sufficient. Applicable law, institutional requirements, participant preferences, and the nature of the research must be considered.
Should Pregnant People Automatically Be Excluded From Research?
No. Pregnancy can introduce important physiological and ethical considerations, but a blanket exclusion is not justified in every study.
In some interventional research, an investigational product or procedure may present uncertain or unacceptable risks during pregnancy. Exclusion may then be appropriate, particularly when available evidence is insufficient to support safe participation and safeguards cannot adequately address the risk.
Other research, including certain observational studies, interviews, educational interventions, and appropriately designed clinical investigations, may involve little or no additional pregnancy-related risk.
CIOMS Guideline 19 addresses pregnant and breastfeeding women as research participants and emphasizes both the importance of research relevant to their health needs and careful assessment of potential risks and benefits.
For studies involving pregnancy, researchers should evaluate the intervention or procedure, available evidence, relevant maternal and fetal considerations, consent requirements, and appropriate monitoring arrangements.
Excluding pregnant people from research that may eventually inform their care can also leave important evidence gaps. The ethical assessment therefore concerns both the risks of participation and the consequences of insufficient research evidence.
What About People With Multiple Health Conditions or Those Taking Several Medications?
Clinical studies sometimes exclude participants with comorbid conditions, organ dysfunction, or concurrent medication use. Such restrictions may be appropriate when they address genuine contraindications, important interactions, or substantial difficulties interpreting outcomes.
However, excluding everyone with any additional diagnosis can create a study population that differs considerably from the patients likely to receive an intervention in routine practice.
For example, a trial of a treatment commonly used by adults with several chronic conditions may produce evidence of limited applicability if the protocol admits only participants without any other health condition.
Rather than using a blanket restriction, researchers can consider whether specific conditions, severity thresholds, medication interactions, or monitoring requirements offer a more scientifically defensible approach.
The United States Food and Drug Administration's December 2025 guidance on enhancing participation in clinical trials discusses eligibility criteria and enrollment practices intended to support the inclusion of a broader range of relevant participants. Its recommendations apply within the guidance's specified regulatory context and should not be mistaken for universal eligibility requirements.
Can Language, Literacy, and Technology Requirements Function as Hidden Eligibility Criteria?
Yes. Some restrictions appear in the formal eligibility section, while others emerge through the practical requirements of participation.
A protocol may state that all adults meeting a particular condition are eligible but require participants to complete an English-language electronic consent form and an online questionnaire independently. These requirements can exclude otherwise eligible individuals.
Researchers should distinguish between genuine measurement requirements and administrative choices. A study evaluating English-language reading comprehension may reasonably require a defined level of English proficiency. A study examining general patient satisfaction may have less justification for excluding people solely because they cannot complete an English questionnaire.
The implications of language-based eligibility restrictions and internet-dependent participation requirements should therefore be examined alongside formal inclusion and exclusion criteria.
Alternative procedures may improve accessibility, although translated instruments and different administration modes may require additional evaluation to establish acceptable measurement comparability.
How Do Eligibility Restrictions Affect Generalizability and Scientific Validity?
Eligibility criteria determine which individuals can contribute evidence. Consequently, they help define the population for which the study's findings are most directly informative.
Restrictive criteria may reduce heterogeneity and simplify some aspects of analysis. In certain explanatory studies, this can support a narrowly defined scientific objective. However, narrower eligibility does not automatically improve internal validity, nor does broader eligibility automatically improve external validity.
The effect depends on the research question, study design, causal assumptions, measurement procedures, and characteristics of the excluded population.
For an intervention study, excluding people whose characteristics modify treatment effects may limit the transportability of findings to the population likely to receive the intervention.
For observational research, exclusions can alter the distributions of exposures, outcomes, and relevant covariates. In some circumstances, conditioning on eligibility can also introduce selection bias.
For qualitative research, restrictive criteria may exclude experiences essential to understanding the phenomenon, although purposive boundaries can be appropriate when they are conceptually justified.
These considerations explain why inclusive design should be evaluated in relation to scientific validity, not treated as a separate administrative concern.
How Can Researchers Justify an Exclusion Criterion?
A defensible justification should identify the precise reason for exclusion and connect it to the study's objectives or participant protection requirements.
Researchers should be able to explain what would happen if the criterion were removed, whether a narrower restriction could achieve the same purpose, and whether additional safeguards could make participation acceptable.
| Proposed restriction |
Question requiring justification |
Possible refinement |
| Exclude everyone older than 65 |
Does age itself determine eligibility, or is a specific clinical characteristic relevant? |
Assess the relevant risk or functional characteristic directly when feasible. |
| Exclude all people with disabilities |
Which actual study requirement cannot be met with appropriate accommodation? |
Specify necessary functional requirements and assess accessible alternatives. |
| Exclude all pregnant participants |
What study-specific pregnancy-related risk or scientific limitation supports exclusion? |
Consider the procedure, available safety evidence, and whether safeguards or a narrower restriction are appropriate. |
| Exclude anyone with a comorbidity |
Which conditions meaningfully affect safety or interpretation? |
Identify specific contraindications or clinically relevant thresholds. |
| Require fluent English |
Is English proficiency part of the construct being measured? |
Consider translated materials or language support where measurement quality can be maintained. |
| Require independent digital participation |
Is independent technology use scientifically necessary? |
Consider accessible interfaces or alternative administration procedures. |
These refinements are options for evaluation rather than automatic replacements. A criterion should be changed only after considering the specific protocol, scientific consequences, and relevant ethical or regulatory requirements.
Can Researchers Broaden Eligibility Without Compromising Participant Safety?
Sometimes. Broader eligibility may be supported through targeted safety assessments, additional monitoring, adjusted procedures, appropriate accommodations, or carefully defined subgroups.
For instance, a study may replace an arbitrary age limit with an assessment of specific contraindications. An interview study may offer accessible communication formats rather than excluding participants who cannot use the original format.
However, inclusion can introduce legitimate resource and safety considerations. Additional monitoring may be expensive, certain procedures may not be appropriate for every participant, and some modifications may change the intervention or measurement.
The appropriate response is to balance inclusive recruitment with participant safety and study feasibility, documenting the reasoning behind the final protocol.
Watch Out
Do not remove a safety-related exclusion solely to increase demographic representation. Eligibility changes in regulated or ethics-approved research may require formal protocol amendments, updated risk assessments, and approval before implementation.
Should Eligibility Criteria Be Reassessed During Recruitment?
Yes, particularly when recruitment patterns reveal that certain relevant groups are rarely eligible or frequently excluded.
Researchers can examine screening logs, reasons for ineligibility, and available demographic or clinical information to determine whether the criteria operate as expected.
These records should be collected and handled in accordance with applicable privacy, consent, ethics, and data protection requirements. Researchers should avoid gathering unnecessary sensitive information merely to document representation.
Where exclusions appear broader than intended, the team can evaluate whether a protocol amendment is justified. Changes should be documented and reviewed through the required approval processes rather than implemented informally.
