Class or level of evidence: epidemiologic basis.

Class of evidence (CoE) is a hierarchical rating system used by EBSJ and most major scientific publications for classifying the overall quality of an individual study. It is a shortcut to identifying what is likely the best (or worst) evidence on a given topic. The “classes” range from I to IV with “CoE I” representing the highest level of evidence, and “CoE IV” representing the lowest level. Assigning a CoE to an individual article is an attempt to provide the reader with a relative assessment of the research study's risk of bias; that is, the likelihood that the results of the study are influenced by various biases rather than the intervention. This article intends to open the eyes of its readership to the many potential confounders and to look behind the claims of CoE 1. Common sources of bias EBSJ considers when critically appraising a study include: Patient selection and allocation of treatment Intention-to-treat analysis Blind or independent assessment for important outcomes Co-interventions applied equally to study groups Patient follow-up rate of less than 85% Adequate sample size Controlling for possible confounding Patient selection and allocation of treatment How patients are selected and allocated for treatment in a clinical study of efficacy and safety is paramount. Ideally, patients are selected based on chance to protect against selection bias and confounding.1 That is why a randomized controlled trial (RCT) is considered the best study design in reducing the risk of bias and achieving a high CoE. It is possible, however, when one conducts an RCT, to still introduce bias into the allocation process. How? Bias can be introduced by allowing those who enroll patients into a study to have access to upcoming assignments. Having access gives the enroller knowledge of the next assignment that could then influence whether a patient is included or excluded based on perceived prognosis. Therefore, care must be taken to ensure that the allocation of the patient to a particular treatment group is concealed; in other words, that the implementation of the random allocation sequence occurs without prior knowledge of treatment assignment.2 Some argue that RCTs that do not provide for proper allocation concealment overestimate the effect of a treatment as much as 30%–40%.3 In the critical appraisal process, one should evaluate whether the allocation was concealed. If it is not reported, be suspicious of potential bias.