03 · What You Need to Know
From the Population You Want to Study to the Population You Can Reach
Start by separating the target population from the accessible population
The target population is the broader group about which your research question seeks to produce knowledge or conclusions. The accessible population is the portion of that population available to the study under its actual circumstances.
For example, suppose your research concerns mathematics teachers in Philippine public secondary schools. That broad group might constitute your target population. If your study can realistically recruit only mathematics teachers from participating public secondary schools in one region during a particular academic year, the population available for your study is considerably narrower.
This distinction is well established in research methodology. Capili describes the accessible population as a geographically and temporally defined subset available for participant recruitment. Other methodological treatments similarly distinguish the population researchers wish to understand from the population or sampling frame from which observations can actually be selected.
Target population
The population your research question ultimately concerns and about which you would like to draw conclusions.
Accessible population
The relevant portion of that population that is realistically available to your study under its actual setting, timeframe, access arrangements, and recruitment conditions.
The two may sometimes be very similar. Often they are not.
Accessibility is not the same as eligibility
A person can satisfy every inclusion criterion and still be inaccessible to your study.
Imagine that your target population is teachers who have used a particular educational platform for at least two years. A teacher in another province may meet that definition perfectly. If you have no recruitment mechanism through which that teacher can be identified or contacted, however, eligibility does not create accessibility.
The reverse problem can occur as well. A university might give you access to all of its students, but only some may satisfy the eligibility criteria for your study.
Eligible
The person or unit satisfies the study's inclusion and exclusion criteria.
Accessible
The study has a legitimate and realistic mechanism through which the person or unit can potentially enter the sampling or recruitment process.
Ultimately, your study usually needs people who are both relevant to the research question and reachable through the sampling and recruitment procedures you can actually implement.
Ask how you would identify members of the population
One of the quickest tests of accessibility is deceptively simple: How would I know who these people are?
For some populations, a usable list or other sampling frame may exist. Examples might include an institutional enrollment list, employee roster, professional register, school directory, membership database, patient registry, or another source appropriate to the study.
A sampling frame is the source from which a sample can actually be drawn. Ideally, it corresponds closely to the target population. In practice, frames may omit some members of the target population, contain units that do not belong to it, or become outdated.
The National Academies' guidance on survey evidence emphasizes that researchers should describe both the target population and sampling frame, examine differences between them, and consider the consequences of those differences. This matters because a frame that systematically excludes parts of the target population can constrain what the resulting sample represents.
Not every research design requires a complete list of every population member, and qualitative or nonprobability sampling may operate differently from probability sampling. The broader principle still applies: you should be able to explain how the people or cases entering your study can actually be found.
Ask whether you are permitted to approach them
Identification does not automatically create access.
A hospital may know exactly which patients satisfy your criteria, but privacy and institutional requirements may prevent you from directly obtaining their contact information. A university may have a complete student database without permitting researchers to email students independently. A company may know which employees perform a particular role while requiring management or another authorized office to approve research contact.
In such situations, access may depend on an intermediary. Authorized personnel might distribute invitations, identify potentially eligible participants, or facilitate recruitment under approved procedures.
This is why a statement such as "the university has 10,000 students" does not establish that 10,000 students are accessible for your study.
When access depends on a particular organization, site, or data custodian, it may be prudent to investigate whether you should contact the potential site or provider before finalizing the research question. The appropriate timing and form of that contact will depend on institutional and ethics requirements.
Geography can define the population you can realistically study
Geographical access is particularly important when participation requires physical presence. Interviews conducted face to face, laboratory measurements, classroom observations, fieldwork, clinical procedures, and equipment-dependent studies may restrict researchers to locations they can realistically visit.
Online data collection can reduce some geographical constraints, but it does not make geography irrelevant in every study. Internet availability, language, time zones, platform access, jurisdictional requirements, and differences among research settings can still matter.
If your research question refers to a national population while your study operates at one local site, you should make that difference explicit rather than allowing the wording of the question to imply broader access than the design actually provides.
Accessibility can also be temporal
A population may be accessible at one time and inaccessible at another.
Students may be available only during the academic term. Seasonal workers may be present during particular months. Patients meeting a specific clinical criterion may appear unpredictably. Employees may become difficult to contact during peak business periods. Archival records may be temporarily unavailable because of digitization, relocation, or administrative restrictions.
Even when a population is routinely available, your study has a recruitment window. If approximately 20 eligible cases become accessible each month and you have three months for data collection, it would be risky to design the project as though the entire annual population were available to you.
Accessibility should therefore be assessed during the period in which the study will actually recruit or collect data.
Access through one site may cover only part of the target population
A common feasibility solution is to recruit from a convenient institution: your university, hospital, workplace, school district, clinic, professional network, or partner organization. This may be entirely appropriate for some research questions and designs.
The methodological issue arises when the accessible site differs meaningfully from the broader population named in the research question.
Suppose you want to understand online learning experiences among university students nationally but recruit only from one private technology-focused university. Those students may differ from students elsewhere in institutional resources, academic programs, socioeconomic circumstances, digital infrastructure, or educational experiences. Increasing the number recruited from that single university does not automatically solve the coverage problem.
Coverage concerns which portions of the target population have an opportunity to enter the sampling process. A large sample drawn from a restricted frame can still omit important segments of the population.
Watch Out
Sample size and population coverage answer different questions. Recruiting many participants from one accessible segment does not by itself make that segment representative of a broader target population that the recruitment frame does not adequately cover.
The gap between target and accessible populations affects inference
The accessible population does not have to be identical to the target population for research to be useful. Many studies necessarily work with a narrower study population. The important question is what the difference means for interpretation.
If inaccessible members are broadly similar, with respect to factors relevant to the research question, to those accessible through your design, the gap may be less consequential. If accessibility systematically favors certain locations, institutions, socioeconomic groups, technologies, health conditions, or other relevant characteristics, broader inference becomes more difficult to justify.
For survey research, the sampling frame should therefore be evaluated against the target population. The National Academies notes that when important groups in the target population are absent from the frame, the survey cannot directly tell us how those unrepresented members would have responded.
This is not merely a technical sampling concern. It can change the substantive meaning of your study.
Convenience is not the same as accessibility
Researchers sometimes use accessible as though it simply meant easy for me to recruit. That is too narrow.
Your classmates, colleagues, students, social-media followers, or employees at your workplace may certainly be easy to contact. Whether they form an appropriate accessible population depends on the research question and sampling strategy.
If your question concerns a population that they reasonably represent or deliberately exemplify under the chosen methodology, their accessibility may be useful. If they are selected merely because they happen to be nearby, convenience has started determining the research question rather than serving it.
The correct sequence is usually to define what evidence the question requires, identify an appropriate population, and then determine which relevant portion can realistically be accessed.
Accessibility is different from successful recruitment
Even a genuinely accessible population will not necessarily produce the sample you need.
You may have permission to approach 1,000 eligible people and still receive only 150 responses. Some may never see the invitation. Others may decline, fail screening, withdraw, or provide incomplete data.
Accessibility therefore answers one feasibility question: Who can realistically enter the recruitment or sampling process? Recruitment feasibility asks the next one: Can enough of those people actually be enrolled and, when necessary, retained?
Once you have established the accessible pool, you still need to determine whether you can recruit the participants your study requires.
Sometimes access uncertainty should change the research question
Suppose your proposed question concerns nurses across ten hospitals, but only one hospital is currently willing to consider participation. You have several options. You might seek additional sites, investigate whether broader access is realistic, redesign the sampling strategy, or narrow the research question to the population you can defensibly study.
What you should avoid is retaining a broad population claim while quietly collecting evidence from a much narrower group simply because that group became available.
A more modest question that accurately reflects the population accessible to the research may be methodologically stronger than an ambitious question whose population exists mostly in the proposal.
