03 · What You Need to Know
How Incentives Affect Retention, Participation Decisions, and Research Validity
What Counts as a Participant Incentive?
Researchers use several forms of financial and nonfinancial support to facilitate participation. Although these arrangements are sometimes collectively described as incentives, they serve different purposes.
Expense reimbursement
Repayment of reasonable participation-related expenses, such as transportation or parking.
Compensation
Payment recognizing the time, effort, inconvenience, or other contributions associated with research participation.
Participation incentive
A benefit intended, at least partly, to encourage enrollment, completion of an assessment, or continued participation.
These categories may overlap. A fixed payment for completing a questionnaire can compensate participants' time while also encouraging response.
Nonfinancial arrangements, such as certificates or access to certain services, may also influence participation decisions. They should not automatically be regarded as ethically insignificant simply because no money changes hands.
Researchers should describe what participants will receive, when they will receive it, and which activities are associated with payment.
What Does Research Evidence Say About Incentives and Retention?
Systematic reviews of retention interventions in randomized trials provide evidence that some incentive arrangements can improve the availability of follow-up data. However, findings vary across incentive types, populations, study settings, and outcomes.
Brueton and colleagues' Cochrane review examined strategies intended to improve retention in randomized trials, including financial incentives and other approaches. The review found evidence supporting certain incentive strategies, while evidence for many other retention interventions was limited or uncertain.
The findings should not be interpreted as demonstrating that any payment amount, incentive schedule, or nonfinancial reward will improve retention in every study.
Evidence from survey methodology also indicates that incentives can affect response behavior, although survey response and longitudinal trial retention are not identical outcomes.
Researchers should therefore seek evidence from studies with comparable participants, assessment demands, and follow-up arrangements.
An incentive that improves completion of a brief postal questionnaire may not have the same effect on repeated clinical assessments or demanding qualitative interviews.
Incentives Can Reduce Practical Barriers to Participation
Some participants face direct costs when completing follow-up assessments.
These may include transportation expenses, mobile data charges, parking fees, childcare arrangements, or time away from paid employment.
Such costs can affect whether individuals are able to continue, even when they remain interested in the research.
Reimbursement or appropriate compensation may reduce these barriers and make participation more feasible.
For example, reimbursing travel expenses may help participants attend follow-up appointments that would otherwise impose substantial financial costs.
However, researchers should distinguish reducing an access barrier from demonstrating that an incentive has corrected selection bias.
Participants may differ in other ways that remain unmeasured, and reimbursement may not resolve scheduling, accessibility, or trust-related barriers.
Incentives Can Change the Composition of the Retained Sample
An incentive may influence some participants more strongly than others.
For example, a payment may be more consequential to someone with limited disposable income than to someone who faces few financial constraints.
Alternatively, an incentive may particularly attract individuals who have sufficient time to complete repeated assessments but would otherwise consider participation insufficiently rewarding.
These differences can change which participants remain available for analysis.
Whether the change reduces or increases bias depends on how incentive responsiveness relates to the study's outcomes and target population.
Consider a study of financial well-being. If compensation changes follow-up participation differently across financial circumstances, the retained sample may differ from the original cohort in characteristics directly relevant to the outcome.
That does not mean compensation is inappropriate. It means researchers should consider its possible selection effects rather than treating improved retention as an unqualified methodological benefit.
Higher Retention Is Not the Same as Lower Attrition Bias
Retention rate describes the proportion of participants who remain available for a specified assessment or other defined research activity.
Attrition bias concerns whether missing observations systematically distort the intended estimate.
These are related but distinct.
Suppose an incentive increases follow-up completion from 75% to 90%. The additional observations may improve precision and reduce the amount of missing information.
However, the remaining 10% may still differ systematically from those observed. The incentive may also have attracted additional participants with particular characteristics.
Conversely, a modest increase in retention could substantially improve an estimate if it reaches a subgroup that was previously underrepresented.
The direction and magnitude of bias cannot be inferred from the retention percentage alone.
This distinction is central when estimating expected attrition during study planning.
Payment Timing Can Influence Participant Decisions
Researchers may provide incentives at enrollment, after individual assessments, at specified milestones, or upon completion of the study.
Each arrangement creates different practical and ethical considerations.
| Payment arrangement |
Potential advantage |
Important consideration |
| Payment after each completed assessment |
Recognizes contributions throughout follow-up |
May influence which assessments participants complete |
| Prorated compensation |
Allows payment for completed activities without requiring full study completion |
Requires clear rules for partial participation |
| Final completion bonus |
May encourage continued follow-up |
Could create pressure to remain enrolled or disadvantage those who withdraw |
| Expense reimbursement |
Reduces direct financial barriers |
Requires transparent eligibility and reimbursement procedures |
| Nonfinancial recognition |
May be suitable for some research contexts |
Value and attractiveness may vary across participant groups |
No arrangement is universally optimal.
The U.S. Food and Drug Administration's guidance on payment and reimbursement to research subjects distinguishes payment arrangements from coercion and emphasizes institutional review of payment amounts and schedules. It also recommends that payment generally accrue as participation progresses rather than being made entirely contingent on study completion.
These recommendations arise in the context of FDA-regulated clinical investigations and should not be presented as a universal legal rule for every research setting.
Nevertheless, they provide a useful framework for considering whether payment arrangements place unnecessary pressure on participants.
Can Incentives Create Undue Influence?
Undue influence concerns circumstances in which an offer may compromise the voluntariness or appropriateness of a participation decision.
Payment is not inherently coercive. An offer of money is different from a threat of harm for refusing participation.
However, substantial incentives may raise concerns when participants are asked to accept significant risks or burdens, particularly if the offer could lead them to discount information relevant to their decision.
The appropriate assessment depends on the study's risks, participant circumstances, payment structure, and applicable ethical requirements.
Researchers should avoid assuming that a particular amount is universally acceptable or unacceptable.
Ethics review should consider the entire arrangement, including what participants must do to receive payment and what happens if they discontinue participation.
Incentives Should Not Make Withdrawal Financially Punitive
Participants should understand whether they will receive compensation for activities already completed if they withdraw before the final assessment.
A payment arrangement that withholds all compensation unless the entire study is completed may create unnecessary pressure to remain enrolled.
Prorated payment can be appropriate in many settings because it recognizes completed participation without making the full amount dependent on continued enrollment.
However, payment procedures must follow the approved protocol and applicable institutional requirements.
Researchers should distinguish compensation already earned from payments associated with activities that have not occurred.
Withdrawal should not be presented as misconduct or as a failure to fulfill an obligation to the research team.
Should Incentives Be Identical Across Study Groups?
In comparative research, differences in compensation may influence follow-up availability.
For example, participants in one intervention group may face additional travel costs because the intervention requires more visits.
Reimbursing those expenses may be appropriate even when the comparison group incurs fewer costs.
However, offering substantially different rewards for completing the same outcome assessment may create unequal retention incentives.
Researchers should distinguish justified reimbursement from differential encouragement to provide outcome data.
The issue is related to maintaining comparable retention opportunities across study groups.
Any group-specific payment arrangements should be specified in the protocol and considered in the interpretation of differential attrition.
Incentives May Affect Responses as Well as Completion
Incentives are commonly evaluated by whether participants complete assessments. Researchers should also consider whether payment arrangements might influence response behavior.
For example, participants may rush through a questionnaire if payment is associated only with submission rather than thoughtful completion.
Others may believe that favorable responses are expected, particularly if compensation is presented by a person with authority over them.
These effects are possible rather than inevitable.
Researchers should avoid linking payment to particular answers or outcomes and should communicate that compensation does not depend on providing favorable responses.
Data-quality procedures should be appropriate to the study and should not penalize participants for legitimate responses that differ from researchers' expectations.
Incentive Effects May Differ Between Recruitment and Retention
An incentive offered at enrollment may influence who initially volunteers.
An incentive introduced at a later follow-up may influence which already enrolled participants provide additional data.
These are different selection processes.
For example, a study that advertises substantial compensation from the beginning may attract a different sample from one that offers modest reimbursement and later introduces an approved follow-up incentive.
Researchers should not assume that evidence about recruitment incentives applies directly to retention incentives.
Changes made during follow-up can also complicate interpretation if they coincide with changes in the participant population or study procedures.
Where possible, incentive arrangements should be established before recruitment and evaluated as part of the retention plan.
Should Researchers Increase Incentives When Retention Falls?
Unexpected attrition may lead researchers to consider increasing compensation.
Before doing so, they should investigate why participants are missing assessments.
If the main barrier is transportation cost, reimbursement may be more appropriate than an unrelated completion bonus. If participants are experiencing questionnaire fatigue, additional payment may not address the underlying burden.
Researchers should also consider whether the change could produce differential participation across study groups or assessment periods.
Material modifications may require ethics approval or a protocol amendment.
The appropriate response should follow the underlying retention problem rather than assuming that more money is always the solution.
How Can Researchers Evaluate Whether Incentives Improve Retention?
Researchers may compare follow-up completion before and after an incentive change or examine differences between participants receiving different approved arrangements.
However, observational comparisons can be misleading.
For example, incentives may be offered preferentially to participants who are already difficult to retain. Their lower completion rate would not establish that incentives are ineffective.
Similarly, completion may improve after incentives are introduced because assessment burden or participant circumstances changed at the same time.
When the effect of an incentive is itself an important research question, an appropriately designed randomized evaluation of ethically acceptable retention strategies may provide stronger evidence.
Such evaluations should consider retention, participant experience, cost, and potential differences in sample composition.
Incentives Should Be Integrated With Other Retention Strategies
Compensation cannot substitute for accessible assessments, respectful communication, realistic scheduling, or appropriate privacy safeguards.
A participant may value an incentive but still be unable to attend an appointment because of work commitments or health circumstances.
Researchers should therefore consider incentives alongside the broader retention strategy specified in the research protocol.
The protocol should identify the incentive amount or form, payment schedule, eligibility conditions, administrative responsibilities, and procedures for participants who discontinue.
It should also establish how payment information will be managed securely and whether collection of identifying financial information is necessary.
These arrangements should be proportionate to the study and consistent with applicable institutional and legal requirements.
Watch Out
Do not use a higher retention rate as proof that an incentive has eliminated bias. Evaluate who remains missing, whether payment changes participation patterns, and whether the arrangement preserves meaningful freedom to decline or withdraw.
