There are several ways to classify the quality of evidence in evidence-based medicine: 1. The U.S. Preventive Services Task Force’s classification can be used to evaluate the quality of evidence for treatment or screening: Level I evidence: evidence from at least one well-designed randomized controlled clinical trial; Level II-1 evidence: evidence from well-designed non-randomized controlled trials; Level II-2 evidence: evidence from well-designed cohort or case-control studies (preferably multicenter studies); Level II-3 evidence: evidence from multiple studies with or evidence from well-designed non-randomized controlled trials; Level II-2 evidence: evidence from well-designed cohort studies or case-control studies (preferably multicenter studies); Level II-3 evidence: evidence from multiple time-series studies with or without interventions. Extremely clear differences in results from uncontrolled trials are sometimes used as evidence for this level; Level III evidence: authoritative opinion from clinical experience, descriptive studies, or expert committee reports. The National Health Service (NHS) in the United Kingdom uses a separate, alphabetical hierarchy of evidence. The American-style grading system above applies only to treatment access interventions. A variety of studies are needed to assess diagnostic accuracy, natural history of disease, and prognosis. Level B evidence: consistent retrospective cohort studies, prospective cohort studies, ecological studies, outcome studies, case-control studies, or extrapolations of Level A evidence; Level C evidence: case series studies or extrapolations of Level B evidence; Level D evidence: expert opinion without critical appraisal, or evidence based on basic medical research. D-level evidence: expert opinion without critical appraisal or evidence based on basic medical research. Overall, the quality of evidence to guide clinical decision making is determined by a combination of the quality of clinical data and the clinical “directionality” of these data. Although there are differences among these evidence grading systems, their purpose is the same: to provide clarity to users of clinical research information about which studies are more likely to be the most effective. In addition, there is a recommended system of evaluation in clinical guidelines and other literature that guides patient-physician communication about medical practices by measuring the risks and benefits of the practice and the level of evidence on which the practice is based. The following are the recommended evaluation criteria from the U.S. Preventive Services Task Force: Level A Recommendation: Good scientific evidence that the benefits of the medical practice substantially outweigh the potential risks. Clinicians should discuss the medical practice with applicable patients; Level B recommendation: At least fair evidence that the benefits of the medical practice outweigh the potential risks. Clinicians should discuss the medical practice with applicable patients; Level C recommendation: At least fair scientific evidence that the medical practice provides a benefit, but the benefit is too close to the risk to make a general recommendation. Clinicians are not required to offer this medical practice unless certain individual considerations exist; Level D recommendation: At least fair scientific evidence suggests that the potential risks of the medical practice outweigh the potential benefits; clinicians should not routinely perform the medical practice on asymptomatic patients; Level C recommendation: The medical practice lacks scientific evidence, or the evidence is of low quality or conflicting, e.g., the risks and benefits cannot be measured and assessed. Clinicians should help patients understand the uncertainty of the medical practice.