Study design
Eligibility criteria and external validity: who a trial can inform
Eligibility criteria translate a research question into a study population. They can protect participants and reduce unwanted variation, but restrictive criteria may leave important uncertainty about people unlike those enrolled.
Educational content only. Not medical advice.
Eligibility defines the population behind the estimate
Inclusion criteria state characteristics participants must have, while exclusion criteria identify conditions that prevent enrollment. Together with geography, sites, recruitment channels, consent, and who accepts participation, they define the population actually represented. A broad condition label in the title can therefore hide a much narrower group in the methods.
Criteria balance safety, measurement, and relevance
Restrictions may protect people at higher risk, avoid interactions, ensure that the outcome can be measured, or create a more interpretable early-development sample. They can also simplify recruitment or increase the chance of detecting a signal. Each rationale has consequences: removing heterogeneity may improve internal precision while reducing information about real-world variation.
External validity is a structured comparison
Generalization requires comparing the trial sample with the target population across age, sex, disease severity, comorbidities, concurrent care, baseline risk, setting, and follow-up. A relative effect may transport differently when baseline risk or effect modifiers differ. External validity is not simply high or low; it is specific to the population and outcome being considered.
Read the flow from screened to analyzed
Eligibility text shows who could enter, while the participant-flow diagram shows who was screened, excluded, randomized, followed, and analyzed. Compare those numbers with registry targets and site locations. If a claim extends to a group that was excluded or scarcely represented, label that extrapolation as uncertain rather than treating the trial average as universal.
Evidence limits
- Published criteria may not capture recruitment barriers or reasons eligible people declined participation.
- Generalizability depends on the target population, setting, outcome, and possible effect modifiers.
- Broad eligibility alone does not guarantee representative enrollment or unbiased follow-up.
Sources and further reading
These sources ground the definitions and evidence boundaries on this page. A citation is a route for verification, not an endorsement of a product or personal use.
ClinicalTrials.gov, U.S. National Library of Medicine
Glossary Terms
Official definitions for trial phases, outcomes, masking, eligibility, adverse events, and study status.
Open sourcePubMed Central
CONSORT 2010 Statement: Updated Guidelines for Reporting Parallel Group Randomised Trials
Peer-reviewed reporting guideline for randomization, allocation, masking, participant flow, outcomes, and analysis populations.
Open sourceU.S. Food and Drug Administration / ICH
E6(R3) Good Clinical Practice Guidance for Industry
Official good-clinical-practice principles for trial design, conduct, randomization, masking, records, and participant protection.
Open sourceCommon questions
Why do trials exclude people with other conditions?
Reasons can include safety, interactions, measurement clarity, or early-development uncertainty, but exclusions limit later generalization.
Is a diverse trial automatically generalizable?
Not automatically. Representation, setting, baseline risk, follow-up, and the specific target population still matter.
Where can I find eligibility criteria?
Check the registry and protocol, then compare them with the enrolled population reported in the publication.
Continue with context
Keep building your evidence-reading skills
Explore the public PeptideSchool research library for more source-backed methods, glossaries, and evidence maps.