• Category I:  Evidence from at least one properly randomized controlled trial.
• Category II-1: Evidence from well-designed controlled trials without randomization.
• Category II-2: Evidence from well-designed cohort or case-control analytic studies, preferably from more than one center or research group.
• Category II-3: Evidence from multiple times series with or without intervention or dramatic results in uncontrolled experiments such as the results of the introduction of penicillin treatment in the 1940s.
• Category III: Opinions of respected authorities, based on clinical experience, descriptive studies and case reports, or reports of expert committees.

Links to websites showing the Hierarchy of Evidence

• Hierarchy of Evidence: University of Sheffield
• Hierarchy of evidence and grading of recommendations, Thorax 2004;59;13-14
• Public Health Information and Data Tutorial: Hierarchy of Evidence