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