Level of Evidence

Introduction

• Evidence-based medicine (EBM) involves using data derived from the scientific method to improve clinical decision-making
• In applying evidence to treat patients, physicians must understand which types of evidence to prioritize
• some evidence is more reliable than others based on study design and quality

Levels of Evidence

• Level 1 (highest)
  • 1a
    • systemic review and/or meta-analysis of high-quality randomized control trials (RCTs) 
      • systemic reviews synthesize previous studies to increase precision and generalizability
      • meta-analysis is the statistical procedure for combining numerical results from synthesized studies 
  • 1b
    • single high-quality RCT  
      • studies in which patients are randomly assigned to the treatment or control group and are followed prospectively
• Level 2
  • 2a
    • high quality non-randomized control trials
  • 2b
    • high quality cohort studies 
      • prospective cohort 
        • studies in which patients are assigned to groups in a non-random fashion by exposure (e.g., treatment or risk factor) with the outcome occurring after the initiation of the study  
      • retrospective cohort
        • a study in which patients are assigned to groups in a non-random fashion by exposure (e.g., treatment or risk factor) with the outcome occurring before the initiation of the study
• Level 3
  • case-control studies
    • studies in which patients are assigned to groups in a non-random fashion by their outcome status (e.g., diseased or not-diseased) and examined for the prior exposure of interest
• Level 4 
  • cross-sectional studies
    • study in which exposure and outcome are assessed simultaneously
  • case series
    • a report of multiple patients with the same disease or treatment but no control group or comparison group
• Level 5
  • case report
    • detailed description of a single patient’s symptoms, signs, treatment, and disease course
  • expert opinion
  • editorials