Hierarchy of evidence
| Evidence level | Study design |
| I | Evidence from systematic reviews or meta-analyses of randomised controlled clinical trials (RCTs). |
| II | Evidence from a well-designed RCT. |
| III | Evidence from well-designed controlled clinical trials without randomization (quasiexperimental). |
| IV | Evidence from well-designed case–control, cohort or cross-sectional studies. |
| V | Evidence from systematic reviews of qualitative and descriptive studies. |
| VI | Evidence from a single descriptive or qualitative study. |
| VII | Evidence from the opinion of authorities and/or reports of expert committees. |