The review protocol should provide explicit, unambiguous, inclusion criteria for the review. Inclusion criteria should be reasonable, sound (based on scientific arguments), and justified. These criteria will be used in the selection process, when it is decided if a study will be included or not in the review. Usually, it is enough to provide explicit inclusion criteria without specifying explicit exclusion criteria; it is implicitly assumed that exclusion is based on the criteria that are the opposite of those specified as inclusion criteria. However, sometimes, for clarity, in order to avoid any potential ambiguity, it is recommended to provide explicit exclusion criteria. Inclusion criteria for a review are not intended to be considered in isolation; in this regard they should be articulated so as to be as mutually exclusive as possible and not repeat information relevant to other aspects of the PICO.
Two categories of inclusion criteria should be considered: inclusion criteria based on study characteristics, and inclusion criteria based on publication characteristics. Inclusion criteria based on study characteristics are those related to the types of participants and settings, types of interventions, comparators, types and measurement of outcomes, and types of studies. Inclusion criteria based on publication characteristics are those related to publication date, language of publication, type of publication (published in commercial scientific databases; documents not published in commercial databases, for example, trials documents). Usually, reviewers use the PICO mnemonic (participants, intervention, comparator and outcome) to construct a clear and meaningful review objective/question regarding the quantitative evidence on effectiveness of interventions. The reviewer uses the same PICO framework to develop inclusion criteria based on study characteristics. The inclusion criteria must provide adequate details about the conceptual and operational definitions of each element to enable reviewers to make reliable decisions when making decisions to include studies.