The criteria for considering types of people included in studies in a review should be sufficiently broad to encompass the likely diversity of studies, but sufficiently narrow to ensure that a meaningful answer can be obtained when studies are considered in aggregate. It is often helpful to consider the types of people that are of interest in two steps. First, the diseases or conditions of interest should be defined using explicit criteria for establishing their presence or not. Criteria that will force the unnecessary exclusion of studies should be avoided. For example, diagnostic criteria that were developed more recently – which may be viewed as the current gold standard for diagnosing the condition of interest – will not have been used in earlier studies. Expensive or recent diagnostic tests may not be available in many countries or settings.
Second, the broad population and setting of interest should be defined. This involves deciding whether a special population group is of interest, determined by factors such as age, sex, race, educational status or the presence of a particular condition such as angina or shortness of breath. Interest may focus on a particular setting such as a community, hospital, nursing home, chronic care institution, or outpatient setting. Box 5.2.a outlines some factors to consider when developing criteria for the ‘Types of participants’.
The types of participants of interest usually determine directly the participant-related eligibility criteria for including studies. However, pre-specification of rules for dealing with studies that only partially address the population of interest can be challenging. For example, if interest focuses on children, a cut-point such as 16 years old might be desirable, but does not determine a strategy for dealing with studies with participants aged from 12 to 18. Use of arbitrary rules (such as “more than 80% of the participants are under 16”) will not be practical if detailed information is not available from the study. A phrase such as “the majority of participants are under 16” may be sufficient. Although there is a risk of review authors’ biases affecting post hoc inclusion decisions, this may be outweighed by a common sense strategy in which eligibility decisions keep faith with the objectives of the review rather than with arbitrary rules. Difficult decisions should be documented in the review, and sensitivity analyses can assess the impact of these decisions on the review’s findings (see Chapter 9, Section 9.7).
Any restrictions with respect to specific population characteristics or settings should be based on a sound rationale. It is important that Cochrane reviews are globally relevant, so justification for the exclusion of studies based on population characteristics should be explained in the review. For example, focusing a review of the effectiveness of mammographic screening on women between 40 and 50 years old may be justified on the basis of biological plausibility, previously published systematic reviews and existing controversy. On the other hand, focusing a review on a particular subgroup of people on the basis of their age, sex or ethnicity simply because of personal interests when there is no underlying biologic or sociological justification for doing so should be avoided. When it is uncertain whether there are important differences in effects among various subgroups of people, it may be best to include all of the relevant subgroups and then test for important and plausible differences in effect in the analysis (see Chapter 9, Section 9.6). This should be planned a priori, stated as a secondary objective and not driven by the availability of data.