Level I: Evidence obtained from at least one properly designed randomized controlled trial.
Evidence from large randomized clinical trials (RCTs) or systematic reviews (including meta-analyses) of multiple randomized trials which collectively has at least as much data as one single well-defined trial.
Evidence from at least one “All or None” high quality cohort study; in which ALL patients died/failed with conventional therapy and some survived/succeeded with the new therapy (for example, chemotherapy for tuber- culosis, meningitis, or defibrillation for ventricular fibrillation); or in which many died/failed with conventional therapy and NONE died/failed with the new therapy (for example, penicillin for pneumococcal infections).
Evidence from at least one moderate-sized RCT or a meta-analysis of small trials which collectively only has a moderate number of patients.
Evidence from at least one RCT.