Level of evidence5
| Ia – Meta-analysis of randomised controlled trials |
| Ib – Randomised controlled trial |
| IIa – Controlled study without randomisation |
| IIb – Quasi-experimental study |
| III – Non-experimental descriptive studies, such ascomparative, correlation and case–control studies |
| IV – Expert committee reports or opinion or clinicalexperience of respected authorities, or all |