Your inclusion/exclusion criteria defines a set of characteristics that are used to determine whether a study should or should not be included in your review. The specifics of your criteria should be driven by your research question (i.e. PICO etc.) so that you are able to target the studies that potentially answer that question.
Thus, the eligibility criteria for your systematic review should be done after you formulate your research question, but before you begin searching for studies.
A clearly outlined inclusion/exclusion criteria is essential for the construction of a comprehensive search strategy.
The elements of your inclusion/exclusion criteria typically reflect the population under study, the intervention/comparator used, and the type of study design. Outcomes are not usually part of your inclusion/exclusion criteria. For more on this, see the Cochrane Handbook.
Source:
McKenzie JE, Brennan SE, Ryan RE, Thomson HJ, Johnston RV, Thomas J. Chapter 3: Defining the criteria for including studies and how they will be grouped for the synthesis. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.3 (updated February 2022). Cochrane, 2022. Available from www.training.cochrane.org/handbook.
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The first element in your inclusion/exclusion criteria is the population under study. It is important that your are clear in defining the characteristics of your population and this requires a balance between being broad enough to allow for a diversity of studies and being narrow enough so the results have meaning.
Your population must be predefined and unambiguous. It should identify and describe the disease or condition and the kind of individuals.
Source:
McKenzie JE, Brennan SE, Ryan RE, Thomson HJ, Johnston RV, Thomas J. Chapter 3: Defining the criteria for including studies and how they will be grouped for the synthesis. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.3 (updated February 2022). Cochrane, 2022. Available from www.training.cochrane.org/handbook.
Identifying and describing the intervention being studied can be straightforward; a test for the efficacy of a drug, is one example.
Other interventions can be slightly more complicated to describe, such as different forms of exercise as an intervention for weight-loss, and will require that your are very clear in your description and identification of what sort of "exercise" you are investigating.
In some instances, you might be including the examination of a comparator intervention in order to show the efficacy of one over another. Again, it is imperative that your description of this comparator be very clearly articulated.
Source:
McKenzie JE, Brennan SE, Ryan RE, Thomson HJ, Johnston RV, Thomas J. Chapter 3: Defining the criteria for including studies and how they will be grouped for the synthesis. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.3 (updated February 2022). Cochrane, 2022. Available from www.training.cochrane.org/handbook.
Your inclusion/exclusion criteria should also include a clear description that justifies the type of studies you will include. The type of research question you are asking will have an impact on the type of studies employed to answer that question. It is essential that you are clear on what designs you will and will not include.
Source:
McKenzie JE, Brennan SE, Ryan RE, Thomson HJ, Johnston RV, Thomas J. Chapter 3: Defining the criteria for including studies and how they will be grouped for the synthesis. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.3 (updated February 2022). Cochrane, 2022. Available from www.training.cochrane.org/handbook.
The decision to include all publication types is one way that your review can show it is reducing the potential for bias. Different publications, published, semi-published, and unpublished material can often be part of a systematic review.
Including grey literature is also encouraged.
Source:
McKenzie JE, Brennan SE, Ryan RE, Thomson HJ, Johnston RV, Thomas J. Chapter 3: Defining the criteria for including studies and how they will be grouped for the synthesis. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.3 (updated February 2022). Cochrane, 2022. Available from www.training.cochrane.org/handbook.
Another way of demonstrating your review is designed to limit bias is to include publications in languages other than English.
Keep in mind, however, that should you decide to widen the scope to include foreign languages, this will require more time and funds to hire translators or someone with advanced reading comprehension.
Source:
McKenzie JE, Brennan SE, Ryan RE, Thomson HJ, Johnston RV, Thomas J. Chapter 3: Defining the criteria for including studies and how they will be grouped for the synthesis. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.3 (updated February 2022). Cochrane, 2022. Available from www.training.cochrane.org/handbook.