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Levels of evidence in neurosurgery.
| Level of evidence | Studies |
| I | (1) Randomized controlled trial, (2) meta-analysis of randomized controlled trials with homogeneous results |
| II | (1) Prospective comparative study (therapeutic), (2) meta-analysis of level II studies or level I studies with inconsistent results |
| III | (1) Retrospective cohort study, (2) case-control study, (3) meta-analysis of level III studies |
| IV | (1) Case series |
| V | (1) Case report, (2) expert opinion, (3) personal observation |
