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Table 1 The hierarchy of evidence from the center for evidence-based-medicine

From: Italian guidelines for the management and treatment of neonatal cholestasis

Level Definition
1a Systematic review (with homogeneity) of randomized clinical trials (RCTs)
1b Individual RCT (with narrow confidence interval)
2a Systematic review (with homogeneity) of cohort studies
2b Individual cohort study
3a Systematic review (with homogeneity) of case–control studies
3b Individual case–control study
4 Case-series (and poor quality cohort and case–control studies)
5 Expert opinion without explicit critical appraisal, or based on physiology, bench research or “first principles”