Meta-analysis or at least 2 randomized controlled trials (RCTs) that included a placebo condition
At least 1 RCT with placebo or active comparison condition
Uncontrolled trial with at least 10 subjects
Anecdotal reports or expert opinion
Levels of evidence do not assume positive or negative or equivocal results, they merely represent the quality and nature of the studies that have been conducted.
Level 1 and Level 2 evidence refer to treatment studies in which randomized comparisons are available. Recommendations involving epidemiological or risk factors primarily arise from observational studies, hence the highest level of evidence for these is usually Level 3. Recommendations, such as principles of care, reflect consensus opinion based on evidence from various data sources, and therefore are primarily Level 4 evidence.