03 · What You Need to Know
Understanding Recruitment Rate as a Feasibility Outcome
Why Is Recruitment Rate Important in a Feasibility Study?
Recruitment is a prerequisite for many research designs. Even a carefully developed intervention, valid measurement strategy, and appropriate statistical analysis cannot compensate for an inability to enroll the participants needed to answer the research question.
Recruitment feasibility becomes particularly consequential when the definitive study requires a specified sample size, a restricted population, multiple institutions, or enrollment within a limited period.
For example, a randomized educational trial may require sufficient classes to achieve its intended statistical precision. If the participating universities cannot recruit enough classes, the proposed design may need modification.
A feasibility study can provide evidence about recruitment under the intended conditions rather than relying exclusively on assumptions.
However, recruitment should not automatically become the primary outcome of every feasibility study. The most important outcome depends on the uncertainties that could prevent the definitive research from proceeding.
What Exactly Is Recruitment Rate?
The term recruitment rate is used inconsistently across research reports. It may refer to the number of participants enrolled during a specified period or to a proportion of individuals moving through a recruitment process.
Researchers should therefore define the measure explicitly instead of assuming that readers will interpret the term in the same way.
| Indicator |
Definition |
Question answered |
| Recruitment speed |
Number enrolled per specified unit of time |
How quickly can participants be enrolled? |
| Screening response proportion |
Number screened divided by number approached |
How many approached individuals enter screening? |
| Eligibility yield |
Number eligible divided by number screened |
How efficiently does screening identify eligible participants? |
| Enrollment proportion among eligible individuals |
Number enrolled divided by number confirmed eligible |
How many eligible individuals agree to participate? |
| Overall recruitment yield |
Number enrolled divided by number approached |
What proportion of approached individuals ultimately enroll? |
These indicators are related, but none should be substituted for another without explanation.
How Do You Calculate Recruitment Speed?
Recruitment speed expresses the number of enrolled participants relative to the recruitment period.
This calculation provides an observed average. It does not establish that enrollment will continue at the same rate.
Recruitment may slow after the most accessible participants have enrolled, fluctuate across academic terms, or differ when additional sites are introduced.
Researchers should also clarify whether recruitment duration includes periods when enrollment was suspended.
How Do You Calculate Recruitment Yield?
Recruitment yield describes the proportion of approached individuals who ultimately enroll, provided that the number approached can be determined reliably.
Recruitment yield can inform the scale of outreach needed for a future study.
However, it may not be calculable when recruitment uses open advertisements and the number of individuals who actually encounter the invitation is unknown.
Researchers should report the available recruitment information rather than invent a denominator.
Why Should Eligibility Yield Be Measured Separately?
Eligibility yield concerns the proportion of screened individuals who satisfy the study's inclusion and exclusion criteria.
For example, a researcher may screen 100 teachers but find that only 35 meet a requirement concerning experience with a particular educational technology.
The eligibility yield is 35%.
A low eligibility yield may suggest that recruitment channels are reaching many individuals outside the target population.
However, it does not necessarily indicate poor recruitment implementation. The target population may genuinely be rare.
Eligibility yield should be interpreted in relation to the research question and the appropriateness of the sampling criteria.
Why Is Enrollment Among Eligible Individuals Important?
Even when enough eligible individuals are identified, some may decline participation.
The enrollment proportion among eligible individuals helps distinguish limited population availability from unwillingness to participate.
For example, if 60 individuals are eligible and 45 enroll, the observed enrollment proportion is 75%.
Researchers may investigate whether nonparticipation relates to workload, study duration, intervention preferences, privacy concerns, or scheduling.
However, a lower enrollment proportion does not automatically indicate a defective recruitment process.
Individuals may decline after making a fully informed decision, and that choice must be respected.
Should Recruitment Rate Be a Primary Feasibility Outcome?
Recruitment rate may be designated a primary feasibility outcome when enrollment capacity represents the central uncertainty motivating the study.
For example, a definitive trial may require participants from a specialized professional population whose availability has not been established.
In that situation, recruitment performance may determine whether the proposed study is realistic.
However, recruitment need not be primary when other uncertainties are more consequential.
A feasibility study of a complex intervention might prioritize delivery fidelity, while recruitment is assessed as a secondary outcome.
The designation should follow the study's objectives rather than a general rule that recruitment must always be primary.
Should Recruitment Rate Be Measured in Qualitative Research?
Recruitment feasibility can also matter in qualitative research, but the interpretation may differ.
Qualitative sampling commonly emphasizes the relevance of participants' experiences, perspectives, or circumstances rather than statistical representativeness.
For example, a study exploring teachers' experiences implementing institutional AI policies may need participants from different institutional contexts.
A feasibility investigation may examine whether these participants can be identified and approached.
However, recruitment speed or enrollment percentage alone does not establish qualitative sample adequacy.
The sampling strategy should remain consistent with the qualitative methodology and research question.
How Does Recruitment Rate Differ in Cluster-Randomized Studies?
Cluster-randomized studies allocate groups, such as schools, classes, or institutions, rather than individual participants.
Recruitment feasibility may therefore need to be assessed at multiple levels.
For example, researchers may measure the number of schools recruited, the number of eligible classes within each school, and the number of students contributing outcome data.
Recruiting many students from very few clusters may not satisfy the statistical requirements of a cluster-randomized design.
Researchers should define recruitment outcomes according to the unit of randomization and the intended analysis.
Cluster size variation and recruitment timing may also require attention.
How Should Recruitment Rate Be Measured Across Multiple Sites?
Multisite studies should consider variation between recruitment locations.
For example, one university may recruit students rapidly while another experiences substantial delays.
An overall recruitment rate can conceal these differences.
Researchers may report recruitment speed and yield by site, alongside contextual information about institutional calendars, access procedures, and participant characteristics.
However, comparisons between sites should account for differences in population size and recruitment opportunities.
Higher recruitment does not automatically indicate better performance when the underlying conditions differ.
What Recruitment Period Should Be Used?
The observation period should be long enough to provide meaningful information about the recruitment process.
A very short recruitment pilot may capture only an initial surge of interest.
For example, students may respond quickly to an announcement at the beginning of a semester, while recruitment slows considerably afterward.
Researchers should consider academic calendars, recruitment cycles, holidays, institutional approvals, and other contextual factors.
The appropriate duration depends on the definitive study's intended recruitment process.
There is no universally sufficient number of recruitment weeks.
Can Recruitment Rate Predict the Required Recruitment Timeline?
Recruitment rate can inform timeline planning, but projections require assumptions.
For example, if a pilot enrolls six participants per week, a simple projection suggests that recruiting 180 participants would require approximately 30 weeks at the same average rate.
However, the projection may be unrealistic if the eligible population becomes depleted or recruitment conditions change.
Researchers should examine whether the pilot period and recruitment sources resemble those expected in the definitive study.
Scenario planning may be more defensible than relying on one point estimate.
How Much Statistical Uncertainty Is Associated With Recruitment Rate?
Recruitment estimates obtained from small feasibility studies may be imprecise.
For recruitment proportions, confidence intervals can help communicate uncertainty, provided the statistical approach and sampling assumptions are appropriate.
Recruitment counts per unit of time may require different approaches, particularly when enrollment varies over time or across sites.
Researchers should not interpret a single observed rate as a guaranteed future value.
Precision should be evaluated according to the decision the estimate must support.
A feasibility study intended to determine whether recruitment can plausibly meet a demanding target may require more information than one designed to identify obvious barriers.
Should Recruitment Rates Be Compared Between Strategies?
Comparing recruitment strategies may be useful when researchers have several possible channels.
For example, recruitment through institutional email may be compared with announcements through a learning management system.
However, the groups reached by these channels may differ.
A higher enrollment proportion does not necessarily mean that the channel is causally more effective.
Fair comparisons require attention to the populations approached, timing, eligibility, costs, and possible selection effects.
Where the objective is to estimate the comparative effect of recruitment strategies, an appropriately designed randomized evaluation may be informative.
Should Recruitment Cost Be Measured Alongside Recruitment Rate?
Often, yes, particularly when recruitment resources could constrain the definitive study.
A strategy may enroll participants rapidly but require substantial staff time or administrative expenditure.
Researchers may calculate recruitment cost per enrolled participant when costs and enrollment are defined consistently.
However, financial cost is not the only relevant consideration.
Recruitment methods may differ in participant burden, accessibility, institutional requirements, and sample composition.
The most efficient strategy is not necessarily the one with the lowest apparent cost.
Can Recruitment Rate Reveal Selection Bias?
Recruitment records may identify differences in who is approached, screened, eligible, and enrolled.
For example, online recruitment may disproportionately reach students who regularly access institutional platforms.
However, recruitment rates alone cannot establish the presence or magnitude of selection bias.
Such an assessment requires information about the intended population and how participation relates to relevant characteristics.
Researchers should examine whether recruitment procedures may systematically exclude groups important to the study's purpose.
Recruitment feasibility and sample adequacy are related but distinct considerations.
How Should Recruitment Progression Criteria Be Established?
Progression criteria specify how recruitment findings will inform the decision to proceed with the definitive study.
For example, researchers may determine the minimum recruitment speed needed to reach the planned sample within the available recruitment period.
That requirement can provide a defensible basis for evaluating feasibility.
However, progression thresholds should be justified by the study's design, resources, and practical constraints.
There is no universal recruitment percentage that establishes feasibility across all studies.
The development of progression criteria is examined further in What Progression Criteria Should Determine Whether a Feasibility Study Leads to the Main Study?.
Should Recruitment Criteria Use Green, Amber, and Red Categories?
A traffic-light approach may help distinguish recruitment outcomes supporting progression from those requiring modification or substantial reconsideration.
For example, green may indicate that recruitment is compatible with the definitive timeline, amber may indicate that improvements are needed, and red may indicate that the current recruitment strategy is unlikely to be workable.
However, the boundaries should be established for the particular study rather than borrowed from arbitrary percentages.
Researchers should also consider uncertainty and whether observed recruitment barriers can be remedied.
A traffic-light classification supports structured judgment but should not replace it.
What If Recruitment Is Slower Than Expected?
Researchers should investigate which stage of recruitment creates the difficulty.
A low enrollment total may result from limited reach, restrictive eligibility criteria, participant concerns, administrative delays, or scheduling difficulties.
Each explanation suggests a different response.
For example, adding recruitment sites may address limited access, while clearer communication may address misunderstandings about participation requirements.
However, changing eligibility criteria or increasing pressure on prospective participants solely to improve enrollment may create methodological or ethical problems.
Modifications should preserve the intended study population and voluntary participation.
What If Recruitment Is Faster Than Expected?
Faster recruitment may support the practicality of the proposed timeline, but it should still be interpreted cautiously.
An initial surge of volunteers may not continue throughout the definitive study.
Researchers should examine whether the recruited sample includes the intended participant groups and whether rapid enrollment creates additional demands for consent, intervention delivery, or outcome assessment.
Recruitment success should not be evaluated independently of the research team's capacity to manage enrolled participants.
Should Recruitment Be Evaluated Together With Retention?
Yes, when the definitive study requires continued participation.
High recruitment may be insufficient if many participants withdraw before the primary outcome is measured.
For example, a longitudinal educational study may recruit 200 students successfully but obtain complete follow-up data from only a substantially smaller group.
Researchers should therefore distinguish recruitment outcomes from retention and outcome completeness.
These measures address different stages of the research process.
For the related issue, see Should Participant Retention Rate Be an Outcome in a Feasibility Study?.
Can Recruitment Rate Establish That the Definitive Study Is Feasible?
Not by itself.
Recruitment may be adequate while intervention delivery, data completeness, participant retention, or resource requirements remain problematic.
For example, a trial may enroll participants quickly but be unable to collect its primary outcome at the required time.
Recruitment rate should therefore be interpreted within the broader feasibility objectives.
The relevant question is whether recruitment capacity, together with other essential conditions, supports the proposed definitive design.
How Should Recruitment Findings Be Reported?
Researchers should report the recruitment period, channels, numbers approached, screened, eligible, and enrolled, where these quantities are available.
Relevant proportions should include clearly defined numerators and denominators.
Recruitment speed should specify the unit of time and any interruptions.
Where appropriate, results should be reported by site or participant subgroup.
The discussion should explain limitations, contextual factors, and implications for the definitive recruitment strategy.
Researchers should avoid presenting a favorable recruitment percentage without explaining how it was calculated.
Watch Out
Do not report recruitment rate as an undefined percentage. Enrollment per week, enrollment among eligible individuals, and enrollment among those approached answer different questions. A feasibility conclusion may be misleading if the numerator, denominator, observation period, or recruitment context is unclear.
