What Should You Do When the Population Exists but You Have No Realistic Route to Reach It?

The participants you need exist, but how are you actually going to reach them?

Sometimes the population for a study is perfectly clear. You know who should participate, why that population fits the research question, and perhaps even where its members are likely to be found.

Then recruitment planning exposes the problem. You do not have their contact information. The institution that holds it cannot release it. A gatekeeper must approve every approach. Relevant organizations will not distribute your invitation. The population is dispersed or hidden. An online community prohibits research recruitment. Or the only apparent route would involve access that you cannot ethically or practically obtain.

The population exists, but existence is not access. If there is no realistic route through which appropriate people can learn about the study and choose whether to participate, the proposed recruitment plan has a fundamental feasibility problem.

Diagnose the access barrier before changing the study

Find out why the population is unreachable before deciding what to change

Start by locating the break in the recruitment pathway

“I cannot access the population” can describe several quite different problems. Treating them as one problem makes it difficult to identify a sensible response.

You might know where potential participants are but lack permission to contact them. You might have permission to recruit within an organization but no mechanism for identifying eligible people. Potential participants may be geographically dispersed, socially hidden, or absent from conventional sampling frames. An intermediary may control communication. Alternatively, the recruitment channel may technically exist but reach so few people that it is practically useless.

This diagnosis matters because each problem implies a different solution. Adding more reminders will not solve the absence of a sampling frame. Negotiating with one gatekeeper will not solve a population that is geographically dispersed across many settings.

Distinguish knowing where people are from having permission to reach them

Researchers sometimes describe a population as accessible because they know which institution, clinic, school, workplace, or organization contains potential participants. That establishes location, not necessarily recruitment access.

Privacy and governance requirements may prevent the organization from giving you names or contact details. An ethics committee or institutional policy may restrict direct approaches. Staff may need to make the first contact. In other cases, the organization may simply decline to facilitate recruitment.

This is why it can be valuable to contact potential sites or data providers before finalizing the research question. A preliminary conversation can reveal what recruitment arrangements might actually be possible, without assuming that access to identifiable information will be granted.

Ask whether an intermediary can provide access without providing identities

Direct access to names and contact details is not always necessary. An institution may be able to distribute an approved invitation to potentially relevant members while keeping their personal information from the research team. Interested individuals can then contact the researchers themselves.

Depending on the study, similar arrangements may be possible through professional associations, community organizations, clinics, schools, support groups, mailing lists, online communities, or other legitimate intermediaries.

The exact arrangement should fit applicable ethical, privacy, organizational, and platform requirements. The important point is that “they cannot give me the participant list” does not automatically mean “there is no recruitment route.”

Gatekeepers can open the route or become the bottleneck

Gatekeepers are people or organizations positioned between researchers and potential participants. They may be institution administrators, clinicians, community leaders, organization officers, service providers, moderators, or others whose cooperation affects recruitment.

Gatekeeping can be necessary and beneficial, particularly where privacy, safeguarding, organizational responsibilities, or community relationships are involved. Research on hard-to-reach populations also shows, however, that gatekeepers can restrict opportunities for potential participants to learn about research. The effects can arise from organizational constraints, limited resources, concerns about burden, mistrust, or judgments about whether potential participants should be approached.

If the entire project depends on one person's or organization's continuing cooperation, that dependency belongs in your feasibility assessment. A study whose participant access is controlled by a powerful gatekeeper carries a different recruitment risk from one with several independent routes.

For hidden or hard-to-reach populations, conventional recruitment may be the wrong tool

Some populations cannot be represented well by a convenient institutional list. Members may be geographically dispersed, socially stigmatized, reluctant to identify themselves publicly, disconnected from formal services, or simply absent from conventional sampling frames.

Research involving rare or hard-to-reach populations may therefore require recruitment approaches adapted to how the population is socially organized and where potential participants can appropriately be reached.

Published literature describes approaches involving community organizations, participant referrals, online or listserv recruitment, multiple sites, and relationship-building with relevant communities. None is universally appropriate. Each approach changes what population can realistically be reached and may introduce sampling limitations that need to be acknowledged.

Do not solve an access problem by quietly changing the population

Suppose your question concerns nurses working in intensive care units, but you cannot obtain access to any intensive care unit. Recruiting whichever nurses happen to be available at your university may make data collection easier, but it does not automatically answer the same question.

This is the point at which convenience can begin to alter the scientific object of the study.

Before substituting a more convenient group, ask whether using a more accessible population changes the scientific question too much. Recruitment convenience is not, by itself, a methodological justification for population substitution.

Multiple sites can help, but only when they are real recruitment possibilities

If one site cannot provide enough access, additional sites may widen the recruitment pool and reduce dependence on a single organization. This can be particularly useful when the population is distributed across institutions.

But writing “additional sites will be recruited if necessary” is not a recruitment strategy unless those sites are plausible. Each additional site may require relationships, approvals, coordination, staff involvement, data arrangements, and time.

A hypothetical backup site should not be counted as recruitment capacity until there is reasonable evidence that it could participate within the study's constraints.

Sometimes the route exists but is not realistic enough to count

An organization might agree to place one flyer on a notice board. An online group may permit a single post. A hospital might allow recruitment only when one extremely busy clinician has time to identify potential participants.

Technically, these are recruitment routes. Practically, they may produce very little access.

This is where the distinction between theoretical and practical access becomes important. A population can be theoretically accessible but practically impossible to recruit if the available route cannot deliver enough appropriate participants within the study period.

When the target population exists entirely behind institutional doors

Common mistakes when the population exists but access does not

Preserve the research question when possible, but do not build a study around imaginary access

An access problem should initially trigger investigation rather than immediate abandonment of the population. Find out whether the barrier belongs to one site, one gatekeeper, one recruitment channel, or the population more generally.

The key is sequencing. Try to solve the access problem without changing the scientific question first. If that proves unrealistic, then make any population or question change explicitly and evaluate what the revised study would actually investigate.

Before declaring the population unreachable, check these possibilities

Questions about research populations you cannot realistically reach

A population you cannot realistically reach cannot support the planned recruitment strategy

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.