Defining the Study Population: Designing Defensible Eligibility Criteria

Purpose of eligibility criteria

Inclusion and exclusion criteria define the target population, participant safety and interpretability. Their purpose is not to manufacture a perfectly homogeneous sample, but to identify a population that can answer the question and remain relevant to practice.

Write operational criteria

Inclusion criteria may specify confirmed diagnosis, age, disease stage, setting, treatment history or measurable baseline status. Exclusion criteria should address safety, major treatment interactions or threats to valid measurement. Avoid subjective phrases such as “appropriate patient” and avoid restating the opposite of every inclusion criterion.

Justify every restriction

• Is it required for safety?

• Does it protect reliable outcome measurement?

• Is a strong treatment interaction expected?

• Is it an ethical or regulatory requirement?

• Does it unnecessarily reduce generalisability?

Older adults, people with comorbidity, pregnant participants, disabled people and those with language differences should not be excluded without a defensible reason.

Transparent reporting

Record how many people were excluded for each reason. Post-hoc eligibility changes can create selection bias and must be documented with timing and rationale.

References

• SPIRIT–CONSORT official guidelines.

• ICH E6(R3) Good Clinical Practice. 2025.

Accessed 20 June 2026.