Evidence-based medicine and levels of evidence

Evidence-based medicine is the practice of making medical decisions based on evidence gained from applying the scientific method. Published studies are evaluated using three key questions: "Are the results valid?"; "What are the results?"; and "Can the results be applied to my patients?" The hierarchy of study methods for obtaining evidence is, in order from least to most useful: laboratory research, editorials, case reports and series, case-control studies, cohort studies, and randomized clinical trials. Retrospective case series can suffer from problems such as selection of a biased sample, mixing of treatment effects, and lack of control group. Randomized clinical trials (and meta-analyses of multiple trials) provide the highest level of evidence because randomization limits confounding and prevents bias of treatment assignment. In addition, randomized trials have standardization of interventions, prospective data collection, and masked outcome measures. Although every question cannot be addressed by a randomized clinical trial, the best available evidence should be sought and used to guide treatments.