A population is a summation of all the organisms of the same group or species, which live in the same geographical area, and have the capability of interbreeding.
In this section, the reviewers should verify the eligibility criteria for inclusion and exclusion of the participants and establish the clinical or radiological characteristics of the patients who will become part of the study. The reviewer must identify if the group is homogeneous, if the characteristics of the patients are representative of those in the clinical question, and if it is acceptable to extrapolate the study data to the clinical question when the characteristics are not similar. The authors have to describe how the confounding factors were controlled (exclusion criteria). For example, some of the variables controlled could be: demographic characteristics (age and sex), habits (smoker, drinker), use of medications (steroids), comorbidities (diabetes mellitus, morbid obesity, neoplasm), general conditions of quality of life (unemployed), pregnancy, previous surgeries, degree of severity of the disease, and early or late presentation. The adequate description of eligibility criteria allows the reproducibility of the study. The questions that need to be answered by the reviewers are the following: 1) are the eligibility criteria for inclusion and exclusion broad and clearly stated; 2) is the condition used in the selection clear, such as tests, scores, signs, and symptoms; 3) is the group homogeneous; 4) are the characteristics of the patients representative of those of the clinical question; 5) are the baseline characteristics reported; and 6) were patients similar at baseline in terms of demographics and comorbidity?.