Hierarchy of evidence.
| I. | Evidence obtained from at least one properly randomized trial. |
| II. | Evidence obtained from well-designed controlled trials without randomization. |
| III. | Evidence obtained from well-designed cohort or case-controlled analytic studies, preferably from more than one center or research group. |
| IV. | Evidence obtained from multiple time series with or without intervention. Dramatic results in uncontrolled experiments (e.g., the results of the introduction of penicillin treatment in the 1940's) could also be regarded as this type of evidence. |
| V. | Opinions of respected authorities based on clinical experience, descriptive studies and case reports, or reports of expert committees. |
| I. | Evidence obtained from at least one properly randomized trial. |
| II. | Evidence obtained from well-designed controlled trials without randomization. |
| III. | Evidence obtained from well-designed cohort or case-controlled analytic studies, preferably from more than one center or research group. |
| IV. | Evidence obtained from multiple time series with or without intervention. Dramatic results in uncontrolled experiments (e.g., the results of the introduction of penicillin treatment in the 1940's) could also be regarded as this type of evidence. |
| V. | Opinions of respected authorities based on clinical experience, descriptive studies and case reports, or reports of expert committees. |