How Do You Know Whether a Recruitment Target Is Realistic Before a Study Begins?

Your Study Requires 400 Participants, but Can You Actually Recruit Them?

You have calculated that your study requires 400 participants with usable data. Your proposal identifies three universities as recruitment sites, and you expect to complete recruitment within eight weeks.

However, you have not confirmed how many students meet the eligibility criteria, how many can be contacted, or how many are likely to participate.

Your adviser asks whether the target is realistic. You respond that the combined universities enroll thousands of students, so recruiting 400 should be possible.

But total enrollment does not establish recruitment feasibility. The relevant question is whether enough eligible and accessible individuals can be approached, enrolled, and retained within the actual recruitment period.

A feasibility assessment examines this problem before the study commits substantial resources to an operationally uncertain plan.

A Realistic Target Must Be Supported by Evidence About Access, Yield, and Time

Evaluating Whether the Required Sample Can Be Recruited

What Does Recruitment Feasibility Mean?

Recruitment feasibility concerns whether a study can obtain its required participants through legitimate and practicable recruitment procedures.

It includes the availability of eligible individuals, permission to contact them, willingness to participate, and the capacity to complete recruitment within the planned period.

Feasibility is not equivalent to statistical sample-size adequacy.

A sample-size calculation may establish that 400 participants are needed for a specified precision or power objective. A recruitment assessment determines whether obtaining those participants is operationally plausible.

Both are necessary for a credible research plan, but they answer different questions.

Why Is the Total Population Not Enough?

A large institutional population does not necessarily contain enough eligible and accessible participants.

For example, a university may enroll 10,000 students, but only 1,000 may have completed the particular online laboratory activity being investigated.

Of those 1,000, some may be unavailable during recruitment, and others may decline participation.

The relevant recruitment pool is therefore smaller than the institution's total enrollment.

Researchers should distinguish population size, eligible population, accessible population, and expected enrolled sample.

What Is the Difference Between Eligibility and Accessibility?

Eligibility concerns whether individuals satisfy the study's inclusion and exclusion criteria.

Accessibility concerns whether researchers can legitimately identify, contact, and recruit those individuals.

For example, students may meet all eligibility criteria but be inaccessible because the research team lacks permission to contact them.

Similarly, eligible teachers may be unavailable during a semester break or unable to participate within the scheduled interview period.

A realistic recruitment plan must consider both conditions rather than assuming that every eligible individual can be approached.

Which Inputs Are Needed for a Feasibility Assessment?

Each estimate should be linked to evidence where possible. Unsupported assumptions should be identified explicitly rather than presented as established recruitment facts.

How Do You Estimate Expected Recruitment Capacity?

A basic feasibility calculation multiplies the number of individuals who can be approached by the expected overall usable-data yield.

This calculation represents an expected yield rather than a guaranteed result.

Researchers should also consider whether the accessible population is finite, whether individuals may be contacted more than once, and whether recruitment capacity changes over time.

How Do You Calculate the Required Recruitment Rate?

Another approach is to determine the average number of usable participants needed per active recruitment week.

The average rate is a planning benchmark. It does not imply that recruitment will occur uniformly.

Some studies experience strong initial responses followed by declining recruitment, while others have slow activation followed by increased enrollment.

Where follow-up data are required, the schedule must also allow sufficient time after enrollment for participants to complete the relevant assessments.

Why Is Recruitment Yield Important?

Recruitment yield describes the proportion of approached individuals who ultimately contribute the required data.

It may reflect screening completion, eligibility, consent, retention, and data usability.

For example, a study with high eligibility but low consent may face different operational problems from a study with high consent but substantial attrition.

Researchers should examine these stages separately where possible.

The effects of eligibility, refusal, and attrition on recruitment targets can substantially change feasibility projections.

What Sources Can Support Recruitment Assumptions?

Relevant evidence may include previous recruitment records from the same institution, comparable studies, approved administrative counts, and preliminary screening results.

Published recruitment rates can provide context but should not be transferred automatically to a different population or study design.

For example, response rates from a short anonymous survey may not predict enrollment in a longitudinal study requiring repeated assessments.

Researchers should consider differences in participant burden, incentives, recruitment channels, and institutional context.

When credible estimates are unavailable, a formal feasibility assessment may be warranted.

How Can You Test Optimistic and Conservative Scenarios?

A single recruitment projection can create false confidence when the assumed yield is uncertain.

Researchers can calculate expected enrollment under several plausible conditions.

The values are hypothetical. They illustrate that a target of 400 may appear feasible under central assumptions but become unattainable under a less favorable yield.

The scenarios should be selected based on plausible recruitment conditions rather than arbitrary labels.

Does Expected Recruitment Capacity Guarantee Success?

No. An expected count does not establish a high probability of meeting the target.

For example, if 800 independent contacts each have a 50% probability of yielding usable data, the expected number is 400. The realized number may nevertheless be below 400.

Under a simplified binomial model, researchers can calculate the probability of achieving at least the required sample.

However, recruitment outcomes may be dependent, and the true yield probability may be uncertain.

Assurance-based planning can therefore be useful when failing to achieve the target would have substantial consequences.

How Does a Finite Accessible Population Affect Feasibility?

If the accessible population is smaller than the number of required contacts, the planned recruitment process cannot achieve its expected target without additional sources of participants or changed assumptions.

For example, a study may need 600 invitations but have permission to approach only 450 distinct eligible individuals.

Repeated reminders to the same individuals may improve response, but they do not create additional eligible people.

Researchers should evaluate whether more sites, a longer recruitment period, or an alternative study design is appropriate.

The situation in which the eligible population is smaller than the required sample requires particular methodological attention.

How Does Recruitment Feasibility Differ Across Study Sites?

Sites may have different eligible populations, participation rates, and recruitment permissions.

A pooled projection can conceal an important shortfall at one institution.

For example, three universities may collectively be able to recruit 400 students, but one may contribute only a handful of participants despite being central to a planned comparison.

The overall target could be achieved while the intended site-level analysis remains inadequately supported.

The site-specific recruitment plan should therefore be evaluated alongside the overall projection.

Should Feasibility Be Evaluated Before Ethics Approval?

Preliminary feasibility work should ideally inform the protocol submitted for ethics review.

Researchers can often use aggregate administrative information, published evidence, or existing recruitment records without contacting prospective participants directly.

However, screening, pilot invitations, or other activities involving identifiable individuals may constitute human-participant research or otherwise require institutional authorization.

Researchers should not assume that labeling an activity a feasibility check exempts it from applicable ethics requirements.

Local policies and the nature of the activity determine what approvals are necessary.

When Is a Pilot or Feasibility Study Useful?

A preliminary feasibility study may be useful when recruitment assumptions are highly uncertain and cannot be established from existing evidence.

It may examine screening procedures, recruitment channels, consent processes, retention, and the practical delivery of study activities.

Eldridge and colleagues (2016) provide guidance for reporting randomized pilot and feasibility trials, emphasizing the importance of feasibility objectives distinct from definitive effectiveness testing.

That guidance is specific to randomized pilot and feasibility designs, but the broader distinction between testing feasibility and testing effectiveness is useful.

A separate recruitment feasibility assessment may be more appropriate than launching a full study based on unsupported enrollment assumptions.

Can Recruitment Feasibility Be Improved Without Changing the Research Question?

Sometimes. Researchers may improve invitation clarity, expand approved recruitment channels, adjust scheduling, or reduce avoidable participation burden.

For example, offering several interview appointment options may improve access without changing eligibility criteria.

However, recruitment improvements must preserve voluntary consent and the approved protocol.

Researchers should not weaken scientifically necessary eligibility criteria or pressure students to participate simply to achieve the target.

What If the Required Sample Is Not Feasible?

An infeasible recruitment target should trigger reconsideration of the study plan before data collection where possible.

Possible responses include expanding the recruitment setting, extending the timeline, improving approved recruitment procedures, or revisiting the research design.

For quantitative studies, researchers may need to reconsider the precision or power objectives and transparently evaluate the implications of a smaller achievable sample.

For qualitative studies, feasibility should be considered alongside methodological adequacy rather than replacing it.

A smaller achievable sample should not be declared sufficient merely because the original target is difficult to obtain.

How Should Recruitment Feasibility Be Monitored During the Study?

Researchers should compare observed recruitment performance with planned assumptions at regular intervals.

Useful measures include contacts issued, screening completion, eligibility, consent, enrollment, retention, and usable data.

The team can estimate whether the remaining accessible population and recruitment period are sufficient to meet the target.

For example, if only 100 usable participants have been obtained after four of eight recruitment weeks, achieving a target of 400 would require another 300 participants in the remaining four weeks.

That implies an average of 75 usable participants per week, which should be assessed against actual recruitment capacity rather than assumed achievable.

How Do You Establish Recruitment Review Points?

Review points can be defined according to calendar milestones, enrollment thresholds, or stages of site activation.

For example, a study may review recruitment after the first two active weeks and at the midpoint of the planned period.

At each review, the team should examine whether deviations arise from insufficient outreach, low eligibility, refusal, attrition, or site delays.

The review should lead to operational decisions that remain consistent with the protocol.

Monitoring is most useful when there is still time to make meaningful adjustments.

How Should Feasibility Be Documented in a Research Proposal?

A proposal should describe the required sample, estimated accessible population, anticipated recruitment yield, recruitment duration, and the sources supporting those assumptions.

Where multiple sites are involved, relevant site-level projections should be reported.

Researchers should also identify important uncertainties and contingency measures.

A transparent feasibility statement is stronger than asserting that recruitment is realistic because the institution has a large student population.

The proposal should make clear which figures are verified counts, which are historical observations, and which remain planning assumptions.

Does Feasibility Establish Scientific Adequacy?

No. Recruitment feasibility concerns what can reasonably be achieved, while scientific adequacy concerns whether the achieved information supports the intended research question.

A study may be easy to recruit but inadequately designed. Another may have a strong scientific rationale but require more resources than are available.

The research plan must address both problems.

Researchers should avoid allowing convenience to determine the sample-size justification without considering the consequences for precision, power, interpretation, or transferability.

Testing Whether 400 Participants Can Be Recruited in Eight Weeks

The target appears feasible on an expected-value basis under the central assumptions, but the conservative scenario reveals a substantial risk.

A defensible conclusion would therefore describe recruitment as conditionally feasible rather than guaranteed. The researcher should also evaluate whether the required participants can be obtained from each site in the proportions necessary for the planned analysis.

Common Misconceptions About Recruitment Feasibility

Test Recruitment Assumptions Before Committing to the Study

Estimate the accessible population, expected usable-data yield, and recruitment period before finalizing the research schedule.

Compare the required sample with both expected recruitment capacity and less favorable but plausible scenarios.

A realistic recruitment target should be supported by explicit assumptions and a credible implementation plan, not merely by the number of people who appear to exist within the broader population.

Before Declaring a Recruitment Target Feasible

Frequently Asked Questions About Recruitment Target Feasibility

Recruitment Feasibility Must Be Demonstrated, Not Assumed

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