A critical appraisal of standard guidelines for grading levels of evidence.

Over the past 30 years, a general consensus has emerged within the medical community regarding the essential role served by grading guidelines in evaluating the quality of evidence produced by a medical research study. Specifically, consensus exists regarding the hierarchy of evidence, where randomized controlled trials (RCTs) are considered the ''gold standard'' followed by nonrandomized controlled trials (non-RCTs) and uncontrolled trials. As guidelines have become more sophisticated, processes have been developed for downgrading poorly conducted studies and upgrading strong studies. Lists of threats to internal validity have been disseminated, thereby assisting reviewers in grading studies. However, despite these many accomplishments, considerable issues remain unresolved with respect to how to evaluate the strength of evidence produced by flawed RCTs versus well-conducted non-RCTs. The purpose of this article is to evaluate existing evidence-based grading guidelines and to offer suggestions for how such guidelines may be improved.