03 · What You Need to Know
Designing a Recruitment Feasibility Assessment That Informs Study Decisions
What Is a Recruitment Feasibility Assessment?
A recruitment feasibility assessment examines whether a proposed study can identify, approach, enroll, and, where necessary, retain enough suitable participants within its practical and ethical constraints.
Its purpose is to investigate the recruitment process rather than answer the definitive substantive research question.
For example, a feasibility assessment for an educational intervention might estimate the proportion of students meeting eligibility criteria, the proportion consenting, and the time needed to enroll participants.
The assessment may also examine institutional permissions, recruitment communication, screening procedures, and staff capacity.
Feasibility should be evaluated against the particular study design rather than an assumed universal recruitment standard.
Is Recruitment Feasibility the Same as Statistical Sample-Size Adequacy?
No. Statistical sample-size planning determines the information required to meet a specified precision, power, or other analytical objective.
Recruitment feasibility evaluates whether the required information can realistically be obtained.
A study might need 400 participants for its planned analysis but have access to only 250 individuals who could plausibly enroll.
The sample-size calculation may be mathematically correct while the recruitment plan remains infeasible.
The assessment of recruitment target feasibility connects these two aspects without treating them as interchangeable.
Does Every Research Study Need a Separate Feasibility Study?
No. The intensity of feasibility assessment should reflect the uncertainty and consequences involved.
A study recruiting from a well-characterized institutional registry may have reliable information about eligible population size and previous participation rates.
In contrast, a study requiring repeated follow-up from a difficult-to-reach population may have little evidence about recruitment or retention.
The second study may benefit substantially from preliminary testing.
Researchers should avoid conducting an elaborate pilot merely because pilot studies are considered desirable. Feasibility work should address identifiable uncertainties that could affect a meaningful design decision.
What Forms Can a Recruitment Feasibility Assessment Take?
| Assessment approach |
What it examines |
When it may be useful |
| Desk-based assessment |
Existing population counts, prior recruitment records, published rates, and operational requirements. |
Relevant evidence is already available and reasonably transferable. |
| Institutional consultation |
Recruitment permissions, contact channels, academic calendars, and site capacity. |
Access depends on institutional cooperation. |
| Preliminary screening assessment |
Eligibility rates and the functioning of screening procedures. |
The proportion of eligible individuals is uncertain. |
| External pilot or feasibility study |
Recruitment, consent, retention, and procedural performance in a separate preliminary study. |
Important processes require empirical testing before the definitive study. |
| Internal pilot |
Early implementation within a definitive study under predefined continuation rules. |
The design permits early feasibility evaluation without necessarily discarding initial study data. |
An internal pilot requires careful prospective design. It should not be used as a retrospective label for recruitment problems discovered after the definitive study has already begun.
What Should the Assessment Measure?
Recruitment feasibility outcomes should correspond to the uncertainties most likely to affect study completion.
These may include the number of potentially eligible individuals, the proportion completing screening, eligibility rates, consent rates, enrollment speed, and retention at required follow-up points.
Researchers may also evaluate the time needed to obtain site permissions, the completeness of recruitment records, and the acceptability of invitation procedures.
Not every measure is necessary for every study. A one-time anonymous survey, for example, may have no longitudinal retention requirement.
The assessment should distinguish operational measures from substantive outcomes intended for the definitive analysis.
How Should Recruitment Rates Be Defined?
Each recruitment rate should have a clearly specified numerator and denominator.
For example, eligibility rate may be defined as the number meeting eligibility criteria divided by the number successfully screened.
Enrollment rate may be defined as the number consenting and entering the study divided by the number eligible and invited.
Overall recruitment yield can be calculated as the number contributing usable data divided by the number initially approached, provided the latter denominator is known.
Nonresponse should not automatically be treated as explicit refusal, and individuals contacted repeatedly should not be counted as distinct people.
Consistent definitions allow feasibility findings to inform the calculation of how many individuals must be approached.
How Can Recruitment Capacity Be Estimated?
One approach is to multiply the number of distinct individuals who can be approached by the anticipated proportion who will ultimately provide usable data.
This calculation is an expected-value projection, not a guarantee. It also assumes the accessible count refers to distinct individuals and that the yield estimate applies to them.
How Do You Estimate Recruitment Speed?
Recruitment speed can be expressed as the number of participants enrolled or providing usable data per active recruitment week.
For example, a preliminary assessment might observe 24 enrollments over three active weeks, equivalent to an average of eight enrollments per week.
That rate may inform planning, but it should not be assumed to remain constant throughout the definitive study.
Recruitment may decline as the most accessible individuals are contacted, or increase after additional approved channels become operational.
The assessment should record the number of active recruitment days and distinguish site preparation from actual recruitment activity.
Why Should Feasibility Criteria Be Defined Before the Assessment?
Predefined criteria reduce the risk of interpreting every observed recruitment result as sufficiently promising after the fact.
For example, a study might specify that continuation requires evidence that the projected recruitment capacity can meet the required sample within the available period.
Other criteria may concern site readiness, acceptable participant burden, or the ability to complete follow-up procedures.
The thresholds should be justified by the definitive study's operational requirements rather than borrowed mechanically from another project.
A feasibility assessment is most useful when its results can inform a decision to proceed, modify, or reconsider the proposed study.
What Are Progression Criteria?
Progression criteria are predefined conditions used to evaluate whether a study should proceed to its definitive phase, require modification, or undergo further feasibility work.
A traffic-light approach is sometimes used in pilot and feasibility research.
| Decision category |
Interpretation |
Possible action |
| Proceed |
Feasibility evidence supports the proposed recruitment process. |
Continue with the planned design, subject to other requirements. |
| Modify |
Recruitment appears potentially feasible but requires identifiable improvements. |
Revise approved procedures and reassess projections. |
| Reconsider |
Substantial barriers make the required recruitment unlikely under the proposed design. |
Redesign, undertake further feasibility work, or discontinue the proposed approach. |
These categories are illustrative. Numerical thresholds should be established for the actual study and should account for uncertainty rather than treating one observed percentage as definitive.
How Does Uncertainty Affect Feasibility Findings?
Rates estimated from small preliminary samples can be unstable.
For example, observing 14 enrollments among 20 eligible individuals gives a 70% enrollment proportion, but that estimate has substantial sampling uncertainty.
Researchers should consider confidence intervals or other appropriate uncertainty assessments rather than assuming the observed rate will repeat exactly.
For a simple independent binomial process, the Wilson interval may be preferable to a basic normal approximation, especially with modest denominators.
However, the assumptions behind any interval must be considered. Convenience recruitment and clustering can limit the interpretation of conventional binomial uncertainty estimates.
Should a Feasibility Study Be Powered to Detect an Intervention Effect?
Ordinarily, the primary objective of a recruitment feasibility study is not to establish definitive intervention effectiveness.
Whitehead and colleagues (2016) discuss sample-size considerations for external pilot randomized trials, emphasizing the relationship between pilot objectives and planning for a future definitive trial.
Estimating recruitment and retention parameters requires different reasoning from testing a definitive treatment effect.
A pilot study may collect preliminary outcome data, but such estimates should not be treated as conclusive evidence of effectiveness.
The feasibility sample should be justified in relation to the precision or decision requirements of the feasibility outcomes.
How Should Multiple Study Sites Be Assessed?
Multi-site feasibility should not rely solely on pooled recruitment rates.
One university may have a large accessible population and high participation, while another may experience substantial delays or low eligibility.
If the definitive study requires representation from each institution, these differences matter even when the overall recruitment projection appears adequate.
Researchers should examine site-specific permissions, contact capacity, recruitment speed, and expected yield.
The planning of recruitment separately by study site can reveal constraints that pooled estimates conceal.
Can Feasibility Results Be Transferred to the Definitive Study?
They may inform the definitive study when the populations, recruitment channels, eligibility criteria, and procedures are sufficiently comparable.
However, participation may change with academic calendars, institutional policy, study burden, or the availability of incentives.
A pilot conducted with highly motivated volunteers may overestimate participation in a broader population.
Researchers should identify differences between the preliminary and definitive conditions and consider their likely consequences.
Feasibility findings are conditional evidence, not guarantees of future recruitment performance.
Can Pilot Participants Be Included in the Main Study?
Possibly, depending on whether the feasibility work was designed as an internal pilot or another approved arrangement.
If the early phase uses the same eligibility criteria, measurements, and relevant procedures, inclusion may be methodologically defensible under a prospective plan.
However, substantial changes to the intervention, instrument, or participant selection process may make combining the data inappropriate.
The decision should be specified in the protocol and comply with applicable ethics requirements.
Researchers should not retrospectively include pilot data merely because recruitment for the definitive study proves difficult.
What Ethical Issues Arise During Recruitment Feasibility Work?
Preliminary screening or recruitment may involve personal information, consent decisions, and interactions with prospective participants.
Researchers should determine whether the planned activity requires ethics review or institutional authorization before beginning.
Using aggregate existing counts may involve different requirements from contacting identifiable students or collecting new individual-level information.
Feasibility work should also avoid unnecessary participant burden, inappropriate pressure, and collection of information not needed for its stated objectives.
Institutional policies and applicable legal requirements should be followed rather than assuming that pilot status provides an exemption.
What Should You Do When Feasibility Is Poor?
Poor feasibility results should prompt examination of the underlying problem.
If the eligible population is too small, additional sites or a revised research question may be needed.
If participation is low, the team may consider clearer invitations, more flexible scheduling, or reduced avoidable burden, subject to approval.
If retention is poor, the study may need changes to follow-up procedures or a reconsideration of its longitudinal requirements.
The purpose of feasibility assessment is not to force the original design to proceed, but to inform whether and how it can be conducted responsibly.
How Should Feasibility Findings Be Reported?
Reports should identify the feasibility objectives, procedures, denominators, observed recruitment outcomes, uncertainty, and resulting design decisions.
The CONSORT extension for randomized pilot and feasibility trials provides reporting guidance for relevant randomized designs.
For other research designs, the applicable reporting framework should be selected according to the methodology.
Researchers should distinguish findings from the feasibility assessment from results addressing the definitive research question.
Transparent reporting allows others to understand which recruitment assumptions were supported, which remained uncertain, and what modifications were made.
