01 · The Question
What If Participants Are Not Being Recruited as Planned?
Your study may look feasible on paper and behave very differently once recruitment begins. Fewer people may be eligible than expected. Eligible participants may decline. Recruitment may work at one site but barely move at another. Weeks can pass while the sample grows much more slowly than the timetable assumed.
The natural reaction is to recruit more aggressively: add sites, broaden eligibility criteria, change incentives, extend the recruitment period, or approach a more accessible population. Some of those responses may be reasonable. Others can quietly change who enters the study and therefore change the evidence the study produces.
The first task is consequently not to find more participants at any cost. It is to determine why recruitment is departing from the plan and which response addresses that problem without creating a more serious methodological or ethical one .
02 · The Short Answer
Diagnose the Recruitment Problem Before Changing the Study
In Brief
When participant recruitment does not go according to plan, identify where the recruitment process is failing, reassess whether the original target remains feasible, and use the least disruptive defensible response before changing eligibility criteria, recruitment settings, or the target population.
Monitor recruitment against the original assumptions and document important changes. If a proposed solution alters who can participate, how they are approached, what they are told, or where they are recruited, consider its implications for sampling, validity, ethics approval, the protocol, and the interpretation of the eventual findings.
03 · What You Need to Know
Recruitment Problems Are Not All the Same Problem
Find the Point at Which Recruitment Is Breaking Down
“We cannot recruit enough participants” is a symptom, not yet a diagnosis. Recruitment can fall short for quite different reasons, and those reasons call for different responses.
What you observe
Possible underlying problem
What to investigate
Very few potential participants are identified
The accessible population is smaller than assumed
Population estimates, site volume, recruitment channels, eligibility assumptions
Many people are screened but few are eligible
Eligibility criteria are restrictive relative to the available population
Which criteria exclude participants and whether those criteria remain scientifically necessary
Many eligible people decline
Participation may be burdensome, unattractive, poorly explained, or inappropriate for the population
Reasons for refusal, consent process, time burden, procedures, accessibility, compensation where applicable
Some sites recruit and others do not
The problem may be site-specific rather than inherent to the study
Site capacity, workflow, staffing, access to eligible participants, implementation differences
Recruitment is steady but too slow
The original recruitment rate or timetable may have been unrealistic
Observed recruitment rate, remaining time, resources, and feasibility of extending recruitment
The people enrolling differ from those expected
The recruitment mechanism may be reaching a different segment of the population
Recruitment source, selection processes, participant characteristics, implications for the target population
This diagnosis matters because adding another recruitment advertisement will not solve overly restrictive eligibility criteria, just as extending the recruitment period may not solve a consent procedure that prospective participants consistently find unacceptable.
Compare Actual Recruitment With the Assumptions Behind the Plan
Recruitment should be monitored rather than evaluated only when the study is already in trouble. SPIRIT 2025 recommends that trial protocols describe expected recruitment rates, the duration of recruitment, plans for monitoring recruitment, and strategies intended to achieve adequate enrolment. Its guidance also recognizes that inadequate enrolment can reduce statistical power, delay a study, increase costs, or contribute to early stopping.
For your own study, compare the actual recruitment pathway with what you originally assumed. How many potentially eligible participants are encountered? What proportion are eligible? How many eligible individuals are approached? How many consent? How quickly does this occur?
That comparison can reveal whether the study is experiencing a temporary delay or a structural feasibility problem. A study recruiting 80% as quickly as expected faces a different decision from one recruiting 10% as quickly as expected with no plausible way to improve.
Fix the Process Before Changing the Population
When possible, begin with interventions that address implementation without changing the scientific definition of who belongs in the study. Depending on the cause, this could involve improving site procedures, reducing avoidable participant burden, clarifying recruitment materials, opening an additional recruitment channel that reaches the same intended population, or extending the recruitment period if resources permit.
Changing inclusion or exclusion criteria requires more caution. Eligibility criteria help define the study population. Removing a criterion merely because it makes recruitment inconvenient may change the population to which the findings apply or introduce participants for whom the original intervention, measurement, comparison, or research question was not designed.
If the people you can realistically recruit are materially different from those originally intended, the problem is no longer simply recruitment. You need to consider what it means when the recruitable population differs from the planned study population .
Adding Recruitment Sites Can Change More Than the Recruitment Rate
Opening additional sites is a common response to slow recruitment, particularly in multisite research. It may be methodologically reasonable, but a new site is not necessarily just another source of identical participants.
Sites can differ in patient or participant characteristics, institutional practices, socioeconomic context, referral pathways, intervention delivery, measurement procedures, or other factors relevant to the study. A new site may improve recruitment while simultaneously increasing heterogeneity or changing the composition of the sample.
Before adding sites, consider whether the new setting still represents the intended study population and whether site differences need to be addressed in study procedures or analysis.
Changing Recruitment Materials or Incentives May Require Approval
Recruitment is also part of the ethical conduct of human-participant research. Advertisements, invitations, compensation arrangements, consent procedures, and methods of contacting prospective participants may fall within institutional or ethics-review requirements.
Do not assume that a recruitment strategy can be changed simply because it does not alter the intervention or primary outcome. Check the requirements that apply to your study before implementing revised materials, new recruitment channels, altered incentives, or changes in how prospective participants are approached.
If the modification affects approved study procedures, determine whether the change requires revisiting ethics approval, the protocol, or preregistration .
Do Not Confuse Recruitment With Retention
Recruitment
Getting eligible individuals into the study according to the study's enrolment procedures.
Retention
Keeping enrolled participants sufficiently engaged so that the required follow-up observations or outcomes can be collected.
A study can recruit successfully and still lose too many participants afterward. Conversely, it can have excellent retention among enrolled participants but struggle to enroll enough people. The remedies differ, so examine where participants are being lost before deciding what to change.
Recruitment Success Is Not Just About Reaching a Number
A study can reach its numerical recruitment target and still have a sampling problem. Suppose a survey reaches the required sample size almost entirely through one recruitment channel even though the intended population spans several groups. The numerical target may be satisfied while the resulting sample poorly represents the population relevant to the research question.
Monitor not only how many people are recruited but, where relevant to the design, who is being recruited and through which pathways. The importance of this depends on the sampling strategy and intended inference. Representativeness is not a requirement of every research design, but the achieved sample should remain compatible with the claims the study intends to make.
Know When Recruitment Has Become a Sample-Size Problem
Slow recruitment and insufficient final sample size are connected but not identical. During recruitment, you still have choices about how to improve enrolment. Eventually, however, time, funding, population availability, or other constraints may make the original target unattainable.
At that point, the question shifts from “How can we recruit better?” to what to do when the planned sample size cannot be reached . That requires reconsidering what the smaller achieved sample means for precision, statistical power where relevant, analysis, and the strength of the conclusions.
04 · A Practical Example
A Recruitment Target Falls Behind After the Study Begins
Hypothetical Example
A study is recruiting only half as quickly as expected
A research team plans to recruit 240 participants over 12 months, requiring an average of approximately 20 participants per month. After the first three months, only 30 participants have enrolled rather than the approximately 60 anticipated.
Observe
Recruitment is averaging about 10 participants per month rather than the planned 20.
Diagnose
Screening records show that enough potentially eligible people are available, but many eligible individuals decline because study appointments conflict with working hours.
Consider the least disruptive response
Rather than immediately broadening eligibility criteria, the team examines whether appointment scheduling can be made more accessible without changing the study's scientific procedures.
Check requirements
The team determines whether revised participant-facing information or recruitment procedures require approval before implementation.
Monitor again
Recruitment is reassessed after the approved change to determine whether the actual rate is moving toward a feasible trajectory.
The important feature of this response is not the specific solution. It is the sequence. The researchers identify the bottleneck before changing the design. If the real problem had instead been an extremely small eligible population, changing appointment times would have accomplished little.
06 · What This Means for You
Treat Recruitment Problems as a Diagnostic Exercise
When recruitment falls behind, calculate what is actually happening before redesigning anything. Compare planned and observed recruitment rates, identify where prospective participants are lost, and determine whether the problem is temporary, correctable, or structural.
A simple decision framework
If enough eligible people exist but few are being approached
Investigate recruitment workflow, site implementation, staffing, and recruitment channels.
If many eligible people decline
Investigate burden, accessibility, communication, consent procedures, and documented reasons for refusal where available and appropriate.
If very few screened individuals are eligible
Reassess population estimates and the scientific justification for the eligibility criteria before considering any amendment.
If recruitment is viable but slower than expected
Evaluate whether extending recruitment or adding appropriate capacity is feasible before changing the study population.
If no realistic recruitment strategy can achieve the original target
Reassess study feasibility and the consequences of a smaller sample rather than continuing indefinitely.
Any substantive modification should also be documented. A future reader should be able to understand what was planned, what recruitment problem occurred, what was changed, and why. That record becomes particularly important when the recruitment response also constitutes a change to the research design after data collection has started .
07 · A Quick Checklist
Before Changing Your Recruitment Strategy, Check These Points
When recruitment falls behind, check:
Compare the actual recruitment rate with the rate assumed in the study plan.
Identify whether the bottleneck occurs during identification, eligibility screening, approach, consent, or enrolment.
Review available reasons why eligible individuals decline participation.
Check whether particular sites, channels, or participant groups account for the recruitment shortfall.
Prefer feasible process improvements that preserve the intended population before altering eligibility criteria or the study population.
Determine whether revised advertisements, incentives, sites, eligibility criteria, or consent procedures require approval before use.
Assess whether the revised strategy could change the composition of the sample or the claims the study can support.
Document important recruitment changes, their rationale, timing, and any implications for the study.
09 · The Bottom Line
Do Not Solve a Recruitment Problem by Accidentally Creating a Different Study
The Bottom Line
When participant recruitment does not go according to plan, diagnose where and why recruitment is failing before changing the strategy, then choose a response that addresses the bottleneck while preserving the study's scientific and ethical integrity as far as possible.
Recruitment plans can sometimes be adapted, but adding sites, changing eligibility criteria, altering incentives, or using new recruitment channels may have consequences beyond enrolment numbers. Monitor those consequences, obtain required approvals, and document substantive changes rather than treating recruitment as a race toward a target number.
11 · Cite this Guide
How to Cite This Guide
This guide is intended to be read, shared, and used in research, teaching, and academic work. If you draw on its ideas, explanations, or other content, please acknowledge the source by citing the guide. Doing so gives appropriate credit and helps your readers locate the original resource.
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