03 · What You Need to Know
Designing a Follow-Up Contact Schedule That Is Effective and Ethical
There Is No Universal Rule for Reminder Frequency
Researchers sometimes seek a standard schedule, such as sending reminders every three days or contacting participants once a week.
Such schedules may be practical in particular studies, but they should not be presented as universally validated recommendations.
The appropriate frequency depends on what participants are being asked to do, how much time they have to respond, and whether additional contact is likely to help.
A brief online questionnaire with a two-week completion window may warrant a different approach from a clinical assessment requiring an appointment or a qualitative interview scheduled several months after enrollment.
Contact frequency may also need to differ according to participants' communication needs, provided the procedures are ethically appropriate and do not compromise the research design.
The objective is not to maximize the number of reminders. It is to provide reasonable opportunities for participants to complete the required assessments.
Distinguish Scheduled Follow-Up From Reminder Contact
Researchers should separate the planned measurement schedule from the communications used to support it.
Scheduled follow-up contact
Communication associated with a planned assessment, interview, visit, or other research activity.
Reminder contact
Additional communication intended to help a participant remember or complete an already scheduled activity.
Contact-information verification
Communication intended to confirm that approved contact details remain usable for future follow-up.
Participant-initiated communication
Contact started by a participant to ask questions, update preferences, reschedule, or discuss continued participation.
These contacts serve different purposes and should not automatically follow the same frequency.
For example, a study may require monthly assessments but only one or two reminders for each missed assessment. The monthly measurement schedule does not imply that researchers should contact participants every day between assessments.
Assessment Windows Should Guide Reminder Timing
The permitted assessment window is a central consideration when scheduling reminders.
Suppose a questionnaire is intended to measure participants' experiences during a particular week. A reminder sent after the measurement window has closed may not recover an equivalent observation.
By contrast, a questionnaire assessing relatively stable characteristics may allow a broader completion window, depending on the research design.
Researchers should establish the earliest and latest acceptable assessment dates before selecting reminder intervals.
Reminder timing should then provide meaningful opportunities to respond within those boundaries.
For example, a two-week response window might accommodate an initial invitation, a reminder near the middle of the window, and a final reminder shortly before closure. This is an illustrative arrangement, not an empirically established optimal schedule.
The appropriate spacing depends on participant circumstances and the scientific importance of the measurement timing.
More Reminders Do Not Necessarily Produce Better Retention
Reminders may help when nonresponse results from forgetfulness, competing responsibilities, or an overlooked invitation.
However, the effect of additional reminders is not necessarily linear.
A second message may reach someone who missed the first, while repeated messages after an explicit refusal provide no legitimate retention benefit.
Additional contact can also create annoyance, communication fatigue, privacy concerns, or perceived pressure.
Evidence reviews of retention interventions in randomized trials indicate that the effectiveness of specific strategies varies, and many approaches have limited or uncertain supporting evidence.
Researchers should therefore avoid assuming that a greater number of contacts will always improve retention.
The more appropriate question is whether an additional contact is likely to address a plausible barrier without imposing disproportionate burden.
Contact Frequency Should Reflect the Participant Population
Different populations may have different communication needs and constraints.
Students may be difficult to reach during examination periods or academic breaks. Working adults may have irregular schedules. Patients experiencing illness may find repeated communication demanding, particularly during periods of treatment or recovery.
Participants with limited digital access may also be unable to respond promptly to electronic reminders.
These circumstances do not justify stereotyping participants or assuming that demographic characteristics determine communication preferences.
Instead, researchers should consider the population's likely circumstances during protocol development and provide appropriate opportunities for participants to indicate their preferences.
Where feasible, participant or community consultation may help identify suitable contact methods and reasonable timing.
Ask Participants About Their Preferred Communication Methods
Contact preferences may influence whether reminders are received, understood, and considered acceptable.
Some participants prefer email because it is less intrusive. Others may find text messages easier to notice. Telephone calls may be useful for certain follow-up activities but uncomfortable or impractical for others.
Researchers can consider asking participants which approved communication methods they prefer and whether particular contact times should be avoided.
Preferences should be collected only when justified and handled in accordance with the protocol and applicable privacy requirements.
For sensitive research, the communication channel may also affect confidentiality. A text message visible on a shared phone could reveal information about participation that the individual wishes to keep private.
The preferred channel should therefore be assessed for both convenience and privacy.
Use a Defined Reminder Sequence Rather Than Improvised Contact
A predefined reminder sequence helps ensure consistent implementation.
For a study using an online follow-up questionnaire, the protocol might specify an initial invitation, one or more reminders at defined intervals, and a clear endpoint after which routine reminders stop.
The schedule should also describe whether alternative communication methods may be used when the initial channel fails.
For example, a telephone call should not automatically follow an unanswered email unless that method is permitted and appropriate for the participant.
Staff should know which messages have already been sent so that participants do not receive duplicate or contradictory reminders.
A documented contact sequence also makes it easier to evaluate whether follow-up procedures were implemented as planned.
What Should a Follow-Up Reminder Say?
A reminder should provide enough information for the participant to recognize the study and understand the requested action.
Depending on the study, it may include the assessment name, completion deadline, estimated time required, and a secure method for accessing the questionnaire or contacting the research team.
The wording should remain respectful and avoid suggesting that participation is compulsory.
For example, a message stating that a follow-up questionnaire remains available until a specified date is generally less pressuring than one implying that the participant is failing the research team by not responding.
Reminders should not reveal sensitive study information through channels that other people may access.
Researchers should also avoid overstating the importance of an individual participant's response or implying that withdrawal will invalidate the entire study.
Nonresponse Is Not the Same as Withdrawal
A participant who does not respond to an assessment invitation may have overlooked the message, experienced a temporary barrier, or decided not to continue.
The research team may not immediately know which explanation applies.
Limited follow-up attempts may therefore be appropriate when permitted by the consent arrangements and approved protocol.
However, an explicit request to withdraw or stop contact is different.
Researchers must follow the applicable withdrawal procedures and respect participants' communication preferences.
Continued contact after a participant has clearly declined may create ethical concerns, even when the study is experiencing substantial attrition.
The distinction should be explained to research staff before follow-up begins.
How Should Researchers Handle Participants Who Miss One Assessment?
Missing a scheduled assessment does not necessarily mean that a participant should be removed from all future follow-up.
In longitudinal research, participants may miss one measurement and return for a later one.
The protocol should specify whether such re-engagement is permitted and how it will occur.
For example, a participant who misses a three-month questionnaire may still be invited to complete the six-month assessment, provided they have not withdrawn and continued contact remains authorized.
Researchers should avoid sending repeated messages about an assessment that can no longer provide valid data.
Instead, they may preserve the opportunity for future measurements while documenting the missed observation.
Such procedures should align with the missing-data analysis plan.
Can Contact Frequency Differ Between Participants?
Some variation may be justified by participant preferences, accessibility needs, or approved follow-up procedures.
For example, a participant who requests email rather than telephone contact may receive communications through a different channel without necessarily receiving a different level of research support.
However, discretionary differences can become problematic when staff contact some participants more persistently because they consider them especially valuable or likely to produce favorable outcomes.
In comparative studies, unequal follow-up intensity may also influence outcome availability or participants' experiences.
Researchers should distinguish reasonable individual accommodation from inconsistent procedures that could affect the study's comparisons.
When considering retention strategies across study groups, the protocol should specify whether contact procedures are intended to be equivalent and justify any differences.
Repeated Contact Can Increase Participant Burden
Researchers often view reminders as low-burden activities because each message takes little time to send.
From the participant's perspective, repeated messages may require attention, create interruptions, or generate anxiety about unfinished research tasks.
This burden may be especially relevant when the study concerns sensitive experiences or when participants are managing substantial personal demands.
Researchers should evaluate the cumulative contact burden rather than considering each message in isolation.
The question of when excessive follow-up contact becomes burdensome deserves particular attention in studies with frequent assessments.
Contact procedures should provide a clear way for participants to communicate preferences, ask questions, or request that further reminders stop.
Contact Schedules Should Be Included in the Protocol
Follow-up communication is part of study implementation and may affect both participant welfare and data completeness.
The protocol should therefore identify the planned contact methods, timing, permitted reminders, response windows, and procedures for unsuccessful contact.
It should also specify how withdrawals and requests to stop communication will be recorded.
These arrangements form part of a documented participant retention strategy.
Where appropriate, a separate standard operating procedure may provide the operational details.
Material changes to contact frequency or methods may require ethics or institutional review, depending on the applicable requirements.
Monitor Whether Additional Contact Is Useful
Researchers can examine the timing of completed assessments relative to invitations and reminders.
For example, they may record how many participants complete a questionnaire before the first reminder and how many complete it afterward.
These observations can help evaluate recruitment operations, but they should not automatically be interpreted causally.
Participants responding after a reminder might have completed the questionnaire later even without that message.
Stronger evidence about the effects of different reminder schedules generally requires an appropriate comparative design, such as a randomized evaluation of ethically acceptable contact strategies.
Researchers should also consider whether additional reminders increase complaints, opt-outs, or other indicators of burden.
The goal is to evaluate both data collection and participant experience rather than focusing exclusively on completion percentages.
Contact Frequency Should Be Proportionate to the Importance of the Outcome
Not every assessment has equal scientific importance.
A missed primary outcome measurement may have greater consequences for the study than a missed optional feedback questionnaire.
However, scientific importance does not authorize unlimited contact.
Researchers may reasonably allocate follow-up resources according to the importance of particular measurements, provided procedures remain ethically appropriate and consistent with the protocol.
For example, the study might provide a broader completion window or additional approved contact opportunities for a critical endpoint.
The appropriate approach depends on the outcome, participant burden, and whether the additional contact could influence the measurement itself.
Watch Out
Do not interpret the absence of a response as permission for unlimited reminders. Establish reasonable contact limits, respect explicit refusals and withdrawal requests, and avoid contacting participants through unapproved channels or third parties.
