Hierarchy of evidence and appraisal of limitations (HEAL) grading system.

Despite more than 30 years of effort that has been dedicated to the improvement of grading systems for evaluating the quality of research study designs considerable shortcomings continue. These shortcomings include the failure to define key terms, provide a comprehensive list of design flaws, demonstrate the reliability of such grading systems, properly value non-randomized controlled trials, and develop theoretically-derived systems for penalizing and promoting the evidence generated by a study. Consequently, in light of the importance of grading guidelines in evidence-based medicine, steps must be taken to remedy these deficiencies. This article presents two methods--a grading system and a measure of methodological bias--for evaluating the quality of evidence produced by an efficacy study.