The eligibility criteria are the distinct features of the systematic review approach. They pre-determine what studies will be deliberately included or excluded from the review by defining inclusion and exclusion criteria. Most elements of eligibility criteria are determined by framing the research questions and defining the key elements of relevant frameworks. But some are specified based on the topic, type and aim of the review. These can include:
The standard frameworks used for defining key elements are useful in further specifying and identifying key elements of the research questions. The standard frameworks can be used as a reminder to further specify what characteristics of the key elements are relevant to answer the review question. Some examples of relevant factors to the key elements are provided below.
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Framework |
Further specifications |
Examples |
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PICO/PECO |
Subject
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e.g., sample size, sample type, subgroups, or subsets, setting, age range, sex, education, race, a pre-existing condition or status (e.g., obesity, toxic content), factors or indicators |
|
Intervention/exposure |
the number of participants, duration, the method of delivery and setting or location |
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Exposure |
e.g., groups or type (e.g., food compound, chemical or biohazard, processing method or technology), the level (e.g., nutrient intake or diet), intensity or severity (e.g., maximum uptake level), and context (e.g., animal, human studies, or both) |
|
|
Comparator |
type: control, placebo, or participants/subject without the characteristic in question (PEO)
|
|
|
PO |
Population
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e.g., sample size, sample type, subsets or subgroups, age range, sex, education, race, a pre-existing condition, or status (e.g., obesity, allergens), local, national, or global scales |
|
Outcome |
specific variables (e.g., nutrient, or foodborne pathogen), factors related to methods of measurement (e.g., sensitivity or detection limits), |
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PIT |
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e.g., sufficient, or necessary timeframe, duration, detection limits, values of reference tests e.g., likelihood ratios |
Methodological and theoretical criteria
Depending on the topic and the research domain, these may include:
Study design and hierarchy of evidence
For humans and animals’ studies, the study designs are ranked based on their likelihood of introducing bias into the review. Randomised Controlled Trials (RCTs) are considered the highest quality of evidence compared to observational studies and are used solely for estimating the effectiveness of deliberate interventions in some clinical and health related contexts. In healthcare related food topics, the hierarchy of evidence set for human studies applies (See supplementary materials) and can guide the eligibility criteria. However, the hierarchy do not equally apply to other disciplines such as food and nutrition related topics that are more complex than clinical topics. They might cover other key aspects related to specific topic domains (e.g., food animals) and contexts (e.g., environmental health or social wellbeing) that are not part of clinical studies. Therefore, it is important to follow the guidelines and practices that are specifically designed for the topics when setting the eligibility criteria for study designs and ensure to use the correct hierarchy that applies for the topic of the review.
The specification of the research question can also set a limit for the settings or location of interest. For example, a certain community, food environment or system might be within the focus of the review question because the precision of the estimates is critical to support any future plans for the subject setting or location.
Depending on the scope and aim of the review one or more study designs may be eligible for inclusion in the review. Additionally, when the review question is for the purpose of updating previous systematic reviews, certain types of evidence other than experimental research is part of eligibility criteria. For example, expert opinions, historical records, current nutrition and food-based regulations and guidelines etc.
These may include:
The decisions and judgements made about each eligibility criterium should be consistent with the aim of the review and be backed by scientific evidence and standard practices of the topic domains. Usually, all items of the eligibility criteria are turned into a checklist questionnaire to facilitate the screening process.