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Table 1 Levels of evidence for primary research question on therapeutic studies

From: Level of evidence in wrist ligament repair and reconstruction research: a systematic review

Level I High-quality RCT with statistically significant difference or no statistically significant difference but narrow confidence intervals. Systematic review of level I RCTs (and study results were homogenous)
Level II Lesser-quality RCT (e.g. 80% follow up, no blinding, or improper randomization). Prospective comparative study. Systematic review of level II studies or level I studies with inconsistent results.
Level III Case-control study. Retrospective comparative study.  
Level IV Case series.  
Level V Expert opinion. Case report. Technical report.