Part 9 · Module 9
Pilot and Feasibility Trials
Design pilot and feasibility trials around progression criteria and uncertainty about conducting a future definitive trial.
Learning outcomes
After completing this lecture, you should be able to:
- Distinguish feasibility questions from preliminary effectiveness testing.
- Select process outcomes and progression criteria.
- Choose internal or external pilot structures.
- Justify sample size for feasibility parameters.
- Report decisions and adaptations transparently.
Overview
Pilot and feasibility trials investigate whether and how a future definitive trial should be conducted. Their primary objectives concern recruitment, consent, intervention delivery, retention, measurement, data systems and uncertainty in design parameters. They are not small underpowered effectiveness trials.
The study question, design, participant pathway, measurements and analysis must be specified as one coherent system. A recognised design label is useful only when the operational details preserve the comparison or prediction that the study is intended to support.[1–3]
Why this matters
A non-significant treatment comparison in a small pilot does not show that an intervention is ineffective, and a large observed effect is unstable. Feasibility work is valuable when it produces a decision: proceed, proceed with modification or stop because a definitive trial is not practicable or justified.
Core concepts
Feasibility asks whether something can be done
Questions include availability of eligible participants, willingness to randomise, intervention fidelity, acceptability, data completeness and site capacity. A pilot trial is a smaller version of the future trial conducted to test its procedures.
Progression criteria convert findings into decisions
Green, amber and red thresholds can guide progression, modification or stopping. Criteria should be interpreted with uncertainty and context rather than as inflexible hypothesis tests.
Internal pilots contribute to the definitive trial
Participants from an internal pilot remain in the final analysis when methods and outcomes are sufficiently unchanged and prespecified criteria are met. External pilots are separate and allow larger modifications.
Sample size follows feasibility precision
Pilot size can be based on precision for recruitment, retention, fidelity or variance estimates, practical site flow and rare operational problems. It is rarely justified by power to test the clinical effect.
Qualitative and process data add explanation
Interviews with participants and staff can identify why recruitment or delivery succeeds or fails. Integration should be planned and should not replace quantitative feasibility denominators.
When to use this approach
Planning feasibility work
- Which uncertainties could make the definitive trial fail?
- Which measurable outcomes represent those uncertainties?
- What progression thresholds are clinically and operationally meaningful?
- Will the pilot be internal or external?
- What modifications would invalidate inclusion in a final trial?
Working through the key design decisions
Decision 1: Which uncertainties could make the definitive trial fail?. This point should be resolved before recruitment, extraction or assignment begins. For pilot and feasibility trials, the protocol should state the operational rule, who applies it, when it is applied and what source documents demonstrate compliance. The investigators should also consider how a different choice would change the eligible population, comparison, timing, measurement or interpretation. A pilot review of realistic records or participant pathways can reveal ambiguities before they become irreversible exclusions or post hoc analytical choices. The decision should be reviewed across sites and subgroups because a rule that works in one clinic may operate differently where referral, resources or disease spectrum differ. Any permitted exception should be explicit, dated and reported with its frequency and likely consequence.
Decision 2: Which measurable outcomes represent those uncertainties?. This point should be resolved before recruitment, extraction or assignment begins. For pilot and feasibility trials, the protocol should state the operational rule, who applies it, when it is applied and what source documents demonstrate compliance. The investigators should also consider how a different choice would change the eligible population, comparison, timing, measurement or interpretation. A pilot review of realistic records or participant pathways can reveal ambiguities before they become irreversible exclusions or post hoc analytical choices. The decision should be reviewed across sites and subgroups because a rule that works in one clinic may operate differently where referral, resources or disease spectrum differ. Any permitted exception should be explicit, dated and reported with its frequency and likely consequence.
Decision 3: What progression thresholds are clinically and operationally meaningful?. This point should be resolved before recruitment, extraction or assignment begins. For pilot and feasibility trials, the protocol should state the operational rule, who applies it, when it is applied and what source documents demonstrate compliance. The investigators should also consider how a different choice would change the eligible population, comparison, timing, measurement or interpretation. A pilot review of realistic records or participant pathways can reveal ambiguities before they become irreversible exclusions or post hoc analytical choices. The decision should be reviewed across sites and subgroups because a rule that works in one clinic may operate differently where referral, resources or disease spectrum differ. Any permitted exception should be explicit, dated and reported with its frequency and likely consequence.
Decision 4: Will the pilot be internal or external?. This point should be resolved before recruitment, extraction or assignment begins. For pilot and feasibility trials, the protocol should state the operational rule, who applies it, when it is applied and what source documents demonstrate compliance. The investigators should also consider how a different choice would change the eligible population, comparison, timing, measurement or interpretation. A pilot review of realistic records or participant pathways can reveal ambiguities before they become irreversible exclusions or post hoc analytical choices. The decision should be reviewed across sites and subgroups because a rule that works in one clinic may operate differently where referral, resources or disease spectrum differ. Any permitted exception should be explicit, dated and reported with its frequency and likely consequence.
Decision 5: What modifications would invalidate inclusion in a final trial?. This point should be resolved before recruitment, extraction or assignment begins. For pilot and feasibility trials, the protocol should state the operational rule, who applies it, when it is applied and what source documents demonstrate compliance. The investigators should also consider how a different choice would change the eligible population, comparison, timing, measurement or interpretation. A pilot review of realistic records or participant pathways can reveal ambiguities before they become irreversible exclusions or post hoc analytical choices. The decision should be reviewed across sites and subgroups because a rule that works in one clinic may operate differently where referral, resources or disease spectrum differ. Any permitted exception should be explicit, dated and reported with its frequency and likely consequence.
The approach should be chosen because its structure answers the research question, not because the available dataset happens to resemble a familiar table. The investigator should state the target population, time origin, comparison, outcome and intended measure before collecting or extracting data.
Design and participant pathway
Pilot and feasibility randomised trial
Core structure: Test trial procedures and intervention delivery on a limited scale using prespecified feasibility outcomes and decision criteria.
Best suited for: Novel recruitment pathways, complex interventions, uncertain adherence, new data systems and multicentre expansion.
Principal threats: Effectiveness-focused interpretation, vague objectives, post hoc progression criteria, inappropriate sample-size claims and failure to document adaptations.
Feasibility can be examined without randomisation when the uncertainty concerns recruitment, measurement or intervention delivery alone. Randomisation is needed when its acceptability, concealment, contamination or group-specific procedures are uncertain. The pilot should resemble the future setting enough for findings to be informative.
Protocol specification
Decisions to prespecify
- List critical uncertainties and feasibility objectives.
- Define denominators for recruitment, retention and fidelity.
- Set progression criteria and decision process.
- Justify pilot size for precision or operational exposure.
- Specify allowed adaptations and internal-pilot conditions.
- Plan qualitative or process evaluation and reporting.
Protocol item 1: List critical uncertainties and feasibility objectives. This item requires a reproducible definition rather than a broad intention. The study manual should identify the responsible role, data field or procedural record, permissible time window, quality check and response to deviations. Where judgement is unavoidable, assessors should receive examples and escalation rules, and a sample of decisions should be reviewed for consistency. The analysis dataset should retain enough provenance to distinguish planned exclusions, measurement failure, participant withdrawal and genuinely absent information.
Protocol item 2: Define denominators for recruitment, retention and fidelity. This item requires a reproducible definition rather than a broad intention. The study manual should identify the responsible role, data field or procedural record, permissible time window, quality check and response to deviations. Where judgement is unavoidable, assessors should receive examples and escalation rules, and a sample of decisions should be reviewed for consistency. The analysis dataset should retain enough provenance to distinguish planned exclusions, measurement failure, participant withdrawal and genuinely absent information.
Protocol item 3: Set progression criteria and decision process. This item requires a reproducible definition rather than a broad intention. The study manual should identify the responsible role, data field or procedural record, permissible time window, quality check and response to deviations. Where judgement is unavoidable, assessors should receive examples and escalation rules, and a sample of decisions should be reviewed for consistency. The analysis dataset should retain enough provenance to distinguish planned exclusions, measurement failure, participant withdrawal and genuinely absent information.
Protocol item 4: Justify pilot size for precision or operational exposure. This item requires a reproducible definition rather than a broad intention. The study manual should identify the responsible role, data field or procedural record, permissible time window, quality check and response to deviations. Where judgement is unavoidable, assessors should receive examples and escalation rules, and a sample of decisions should be reviewed for consistency. The analysis dataset should retain enough provenance to distinguish planned exclusions, measurement failure, participant withdrawal and genuinely absent information.
Protocol item 5: Specify allowed adaptations and internal-pilot conditions. This item requires a reproducible definition rather than a broad intention. The study manual should identify the responsible role, data field or procedural record, permissible time window, quality check and response to deviations. Where judgement is unavoidable, assessors should receive examples and escalation rules, and a sample of decisions should be reviewed for consistency. The analysis dataset should retain enough provenance to distinguish planned exclusions, measurement failure, participant withdrawal and genuinely absent information.
Protocol item 6: Plan qualitative or process evaluation and reporting. This item requires a reproducible definition rather than a broad intention. The study manual should identify the responsible role, data field or procedural record, permissible time window, quality check and response to deviations. Where judgement is unavoidable, assessors should receive examples and escalation rules, and a sample of decisions should be reviewed for consistency. The analysis dataset should retain enough provenance to distinguish planned exclusions, measurement failure, participant withdrawal and genuinely absent information.
A protocol should make these decisions before outcome-dependent choices become tempting. It should also distinguish the design population from the analysis population, document exclusions after enrolment or sampling, and explain how missing or indeterminate observations will be handled. Any deviation should be dated, justified and retained in the study record.[2–4]
Measurements, timing and analysis interface
Feasibility outcomes should use operational definitions: proportion eligible who are approached, proportion approached who consent, time from eligibility to randomisation, proportion receiving each component, contamination, completeness of primary outcome and site activation time. Confidence intervals communicate uncertainty around these estimates.
The analysis should reflect how participants or clusters entered the study, how observations are related, and how the effect or performance measure was defined. Statistical adjustment cannot repair a comparison that was invalid at the design stage. Conversely, a sound design can still be undermined by an analysis that ignores matching, clustering, censoring, sampling fractions or prespecified thresholds.
Worked example
Applying progression criteria
Green criteria are at least 70% approached, at least 45% consent, at least 85% complete linkage and less than 10% contamination. Amber values trigger specified changes to staffing or reminder delivery. Red values prompt redesign or stopping. Recruitment reaches 68% with clear weekend staffing gaps, consent is 52%, linkage is 94% and contamination is 6%; the oversight group recommends progression after adding weekend screening.
The example should be interpreted in the population and time frame created by the design. The numerical result does not by itself establish causation, transportability or clinical usefulness; those conclusions require additional assumptions and evidence.
Appraising methodological quality
Questions for critical appraisal
- Objectives: Were feasibility questions primary?
- Denominators: Can recruitment and retention pathways be reconstructed?
- Criteria: Were progression thresholds prespecified and interpreted with uncertainty?
- Setting: Do pilot sites represent future implementation?
- Decision: Were modifications and reasons documented?
A reporting guideline can improve transparency, but it is not a substitute for sound design. Appraisal should ask whether the reported methods could generate an unbiased, clinically meaningful answer—not merely whether every checklist item has been mentioned.[3–6]
Suggested reporting
Reporting should identify the design in the title or abstract when appropriate, describe the source and flow of participants, define all key variables, report missing and indeterminate observations, and present estimates with uncertainty. The direction and denominator of every comparison must be explicit.
Estimating nuisance parameters cautiouslyMMed extension
Pilot estimates of standard deviation, ICC or event proportion are imprecise, especially with few clusters or events. Conservative upper confidence limits, external evidence and blinded internal sample-size review may be preferable to plugging a single unstable estimate into the definitive calculation.
Common mistakes
Knowledge check
What is the main purpose of a pilot trial?
Answer and reasoning
To reduce uncertainty about the design and conduct of a future definitive trial.
Should a pilot be powered for treatment efficacy?
Answer and reasoning
Usually no. Its sample size should address feasibility objectives and precision.
What distinguishes an internal pilot?
Answer and reasoning
Its participants can contribute to the definitive trial under prespecified conditions and sufficiently stable methods.
Can a pilot be successful if the trial stops?
Answer and reasoning
Yes, if it reliably shows that the definitive trial is infeasible or needs fundamental redesign.
Key take-home messages
Key take-home messages
- Make feasibility objectives primary.
- Use explicit denominators and progression criteria.
- Justify size through feasibility precision and exposure.
- Interpret treatment effects as exploratory.
- Document the decision and all modifications.
References
- Eldridge SM, Chan CL, Campbell MJ, et al. CONSORT 2010 statement: extension to randomised pilot and feasibility trials. Pilot Feasibility Stud. 2016;2:64. https://doi.org/10.1186/s40814-016-0105-8
- Hopewell S, Chan AW, Collins GS, et al. CONSORT 2025 statement: updated guideline for reporting randomised trials. BMJ. 2025;389:e081123. https://doi.org/10.1136/bmj-2024-081123
- Butcher NJ, Monsour A, Mew EJ, et al. SPIRIT 2025 statement: updated guideline for protocols of randomised trials. BMJ. 2025;389:e081477. https://doi.org/10.1136/bmj-2024-081477
- Eldridge SM, Lancaster GA, Campbell MJ, et al. Defining feasibility and pilot studies in preparation for randomised controlled trials. PLoS One. 2016;11(3):e0150205. https://doi.org/10.1371/journal.pone.0150205
- Avery KNL, Williamson PR, Gamble C, et al. Informing efficient randomised controlled trials: exploration of challenges in developing progression criteria for internal pilot studies. BMJ Open. 2017;7:e013537. https://doi.org/10.1136/bmjopen-2016-013537
- Thabane L, Ma J, Chu R, et al. A tutorial on pilot studies. BMC Med Res Methodol. 2010;10:1. https://doi.org/10.1186/1471-2288-10-1
These materials support learning and research planning. They require adaptation to the study context and do not replace methodological, statistical, ethical, or regulatory consultation.