How-to · Step-by-step
Inclusion and exclusion criteria
Inclusion and exclusion criteria are the pre-specified rules that decide which studies are eligible for a review, defining eligibility by population, study design, timeframe, language and more.
The step most authors miss
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Step by step
How to do it
1.Derive criteria from your research question
Use a structured frame such as PICO to turn each element of your question into an eligibility rule — the population, intervention, comparison and outcomes of interest each define what a study must address to qualify.
2.Define the population and setting
State precisely whose studies count — age range, condition, setting or other characteristics. For example, you might include adults over 65 in community settings and exclude hospital inpatients, depending on your question.
3.Specify the eligible study designs
Decide which designs answer your question. A review of effectiveness might include only randomised controlled trials, while a broader review might admit cohort and qualitative studies. Excluding designs that cannot address the question keeps the review focused.
4.Set limits on dates, language and publication type
Decide a date range (perhaps from a landmark study or guideline forward), which languages you can handle, and whether to include grey literature, conference abstracts or only peer-reviewed work. Justify limits, since each can introduce bias.
5.Write inclusion and exclusion criteria as matched rules
Express the criteria as clear, applicable statements, with exclusion criteria as the explicit counterpart of inclusion criteria. Avoid vague wording so that two reviewers would classify the same study the same way.
6.Pilot and apply the criteria consistently
Test the criteria on a sample of records and refine any that prove ambiguous, then apply them to titles, abstracts and full texts. Recording the number of studies excluded at each stage and the reasons supports a transparent PRISMA-style flow diagram.
Why pre-specify eligibility
Inclusion and exclusion criteria are the gatekeepers of a review. Defining them before screening — ideally in a protocol — does two things. It makes screening consistent, so two reviewers apply the same rules and reach the same decisions, and it makes the process transparent and reproducible, so a reader can see exactly why studies were kept or dropped. Most importantly, pre-specifying eligibility guards against bias: deciding the rules after seeing the studies invites cherry-picking those that support a desired conclusion. A structured question frame such as PICO is a natural source of criteria, since each element suggests an eligibility rule.
Common questions
FAQ
What is the difference between inclusion and exclusion criteria?+
Inclusion criteria are the characteristics a study must have to be eligible for a review — for example a specified population, study design or date range. Exclusion criteria are characteristics that rule a study out, often the explicit counterpart of the inclusion criteria. Together they define the boundary of the review, and a study is kept only if it meets all inclusion criteria and triggers none of the exclusions.
Why should I set the criteria before searching?+
Setting eligibility criteria in advance — ideally in a written protocol — keeps screening consistent between reviewers and protects against bias. If you decided what to include after seeing the studies, you could, knowingly or not, select those that support your preferred conclusion. Pre-specification makes the selection transparent and reproducible, which is a hallmark of systematic reviews.
Should I limit my review by language or date?+
You can, but justify each limit, because both can introduce bias. Excluding non-English studies may omit relevant evidence and skew findings — a concern known as language bias. A date limit is reasonable when a guideline, technology or definition changed at a known point, but an arbitrary cut-off risks missing important work. State and explain any limits you set.
