Modified level of evidence scale for NI research
| Level of evidence | Study type |
|---|---|
| 1 | Systematic reviews or meta-analyzes of randomized controlled trials or individual randomized controlled trials |
| 2 | Systematic reviews or meta-analyzes of predominantly prospective studies, or individual prospective studies |
| 3 | Systematic reviews or meta-analyzes of predominantly retrospective studies, or restrospective case-control studies |
| 4 | Retrospective non-case-control studies of 10 or more patients |
| 5 | Case reports, case series of 9 patients or less, national or state retrospective database studies, animal studies, or other non-human studies |