Types of studies at each level vary somewhat with the clinical question (eg, of diagnosis, treatment, or economic analysis), but typically consist of the following: Level 1 (the highest quality): Systematic reviews or meta-analyses of randomized controlled trials and high-quality, single, randomized controlled trials Level 2: Well-designed cohort studies Level 3: Systematically reviewed case-control studies Level 4: Case series and lesser-quality cohort and case-control studies Level 5: Expert opinion not based on critical appraisal, but based on reasoning from physiology, bench research, or underlying principles For EBM analysis, the highest level of evidence available is selected.