Should You Decide Which Populations Matter Before Searching?

Do You Need to Define the Population Before You Search?

Population boundaries can look obvious until real studies appear. You may intend to study university students, for example, and then encounter research combining university and secondary-school students. Another paper includes mostly undergraduates but a small number of postgraduate students. A third studies students and instructors together without reporting their results separately.

Suddenly, “university students” is not quite as precise as it seemed.

For a structured review, these decisions are better considered before individual studies begin competing for inclusion. Yet defining the population in advance does not necessarily mean translating every demographic, geographic, diagnostic, or contextual characteristic into a database search restriction.

Define the Population Early, but Search It Strategically

Your Population Should Follow From the Research Question

Population is more than an age group or demographic label

In evidence synthesis, the population describes the people, groups, organizations, communities, or other units to which the question applies. Exactly what needs to be specified depends on the question.

For one review, “undergraduate students” may be sufficient. Another may require students enrolled in nursing programs, first-year students, adults with a particular diagnosis, schools serving a defined age range, or institutions operating within a particular educational setting.

Cochrane guidance treats participant characteristics as an important component of review eligibility criteria and recommends describing restrictions involving characteristics such as age, diagnostic criteria, location, and setting when they matter to the review.

The principle is not to describe every imaginable characteristic. It is to specify characteristics that determine whether evidence actually addresses the question.

Ask which population differences could change the meaning of the evidence

A useful population criterion has a reason for being there.

If your question concerns generative AI use among university students, restricting eligibility to higher education may be justified because educational tasks, institutional expectations, assessment practices, and learner autonomy can differ substantially from primary or secondary education. If your question concerns all students' perceptions of generative AI, an undergraduate-only restriction would need a different justification.

The same logic applies to age, diagnosis, profession, educational level, geographic location, socioeconomic characteristics, or other population features. Do not add a restriction merely because it makes the population look methodologically tidy.

Population eligibility and population search terms are different decisions

This distinction is fundamental.

Suppose your review concerns university students aged 18 or older. Adult status may matter for eligibility, but adding an age restriction to the database search could be unnecessary if the higher-education concept already retrieves the relevant literature and age can be checked during screening.

The same issue arises with location, diagnostic subgroups, socioeconomic characteristics, and other features that may not be reliably represented in titles, abstracts, or database indexing.

This is why deciding which concepts should actually become search terms requires a separate judgment from deciding which concepts matter to the research question.

Mixed populations require a rule

One of the most predictable complications is a study containing both eligible and ineligible participants.

Imagine that your review concerns adolescents aged 13 to 17, but a study includes participants aged 12 to 19. Or your review concerns university students, while a survey includes both students and faculty members.

There is no single rule that works for every review. You might include a mixed-population study when data for the eligible subgroup are reported separately. In other circumstances, you might establish a threshold for the proportion of eligible participants, seek subgroup data from the authors, or exclude studies from which the relevant population cannot be isolated.

What matters is deciding how these cases will be handled consistently. Cochrane reporting guidance specifically recommends explaining how studies containing only a subset of eligible participants were addressed.

Setting may be part of the population question

Population and setting are conceptually distinct, but in practice they can interact. “Nurses” could mean hospital nurses, community nurses, nursing faculty, school nurses, or nurses working across several settings. “Students” could refer to primary school, secondary school, university, vocational education, or online professional education.

If setting changes whether the evidence answers your question, define it. If it does not, imposing a setting restriction may unnecessarily narrow the evidence base.

This is where defining what counts as relevant before seeing the search results becomes useful. Population and setting boundaries can then be justified by the intended scope rather than improvised around individual papers.

Do not define the population more narrowly than the question requires

Specificity can feel like rigor. It is not automatically rigor.

Suppose your question concerns university students' experiences with online learning. Restricting the review to students aged 18 to 22, enrolled full-time, studying at public universities, and living on campus might produce a beautifully precise eligibility statement. It might also exclude substantial evidence without improving the answer.

Population restrictions should therefore be defensible in substantive terms. An arbitrary restriction can make a review easier to manage while making its conclusions less informative or less applicable.

This is one mechanism through which overly strict criteria can hide important evidence.

Exploratory searching may legitimately precede final population boundaries

If you are entering an unfamiliar literature, you may not yet know how researchers define the population. An exploratory search can reveal common age ranges, diagnostic definitions, educational levels, terminology, and patterns of mixed samples.

That does not contradict the principle of prespecification. It means the exploratory phase is being used to develop a defensible question before the final structured search begins. Systematic review methodology relies on prespecified eligibility criteria, but question development itself can require substantial preliminary work. Cochrane emphasizes that well-formulated questions guide eligibility criteria, searching, data collection, synthesis, and presentation.

How Population Boundaries Affect a Literature Search

The important decision occurred before screening: the population was defined according to the question. Individual papers then had to satisfy that definition rather than quietly redefining it.

Common Mistakes When Defining a Study Population

Define the Population You Need, Not the Narrowest Population You Can Imagine

Before a structured search, describe the population that must be represented for a study to answer your question. Then examine each proposed restriction and ask what methodological or substantive purpose it serves.

After defining eligibility, make a second decision about retrieval. Which population characteristics need to be represented in the search itself, and which can be assessed during screening?

The same logic applies to other boundaries. You should also consider which study designs can answer the question and whether particular outcomes need to be defined before searching.

Before Finalizing Your Population Criteria

Questions About Population Criteria in Literature Searches

Define Who the Evidence Needs to Represent

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