Categorisation of the quality of evidence
| Quality of evidence | Description |
| I | Evidence obtained from at least one properly randomised, controlled trial |
| II-1 | Evidence obtained from well-designed controlled trials without randomisation |
| II-2 | Evidence obtained from well-designed cohort or case–control analytical studies, preferably from more than one centre or research group |
| II-3 | Evidence obtained from comparison between times and places with or without the intervention, or dramatic results in uncontrolled experiments |
| III | Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees |