Hierarchy of evidence
| Level I | Evidence based on systematic reviews or meta-analyses of RCTs; clinical guidelines based on systematic reviews or meta-analyses |
| Level II | Evidence collected on well-designed RCTs |
| Level III | Evidence from non-RCTs |
| Level IV | Evidence from well-designed cohort and case-control studies |
| Level V | Evidence from systematic reviews of qualitative and descriptive studies |
| Level VI | Evidence from qualitative and descriptive studies |
| Level VII | Evidence from expert opinions and/or reports of expert committees |
RCTs, randomised controlled trials.