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Table 1 Hierarchies of evidence for questions of therapy, prevention, aetiology or harm [CEBM]

From: The effectiveness of physiotherapy treatment on balance dysfunction and postural instability in persons with Parkinson’s disease: a systematic review and meta-analysis

Level 1a: Systematic reviews (with homogeneity) of randomized controlled trials (RCTs)
Level 1b: Individual RCTs (with narrow confidence interval)
Level 1c: All or none studies
Level 2a: Systematic reviews (with homogeneity) of cohort studies
Level 2b: Individual cohort study or low quality RCTs (e.g. <80 % follow-up)
Level 2c: “Outcomes” Research; ecological studies
Level 3a: Systematic review (with homogeneity) of case-control studies
Level 3b: Individual case-control study
Level 4: Case-series (and poor quality cohort and case-control studies)
Level 5: Expert opinion without explicit critical appraisal, or based on physiology, bench research or ‘first principles’