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.