Introduction A method utilized in the 3rd step of evidence based medicine (EBM) to determine the clinical value of a study five steps of EBM formulate an answerable question gather the evidence appraise the evidence implement the evidence evaluate the process See details of Clinical Design Trials Different Levels of Evidence Different Levels of Evidence Level 1 1. Randomized controlled trial (RCT)• a study in which patients are randomly assigned to the treatment or control group and are followed prospectively2. Meta-analysis of randomized trials with homogeneous results Level 2 1. Poorly designed RCT• Follow up less than 80%2. Prospective cohort study (therapeutic)• A study in which patient groups are separated non-randomly by exposure or treatment, with exposure occurring after the initiation of the study3. Meta-analysis of Level 2 studies Level 3 1. Retrospective cohort study• A study in which patient groups are separated non-randomly by exposure or treatment, with exposure occurring before the initiation of the study2. Case-control study• A study in which patient groups are separated by the current presence or absence of disease and examined for the prior exposure of interest3. Meta-analysis of Level 3 studies Level 4 1. Case series• A report of multiple patients with the same treatment, but no control group or comparison group Level 5 1. Case report (a report of a single case)2. Expert opinion3. Personal observation JBJS Level of evidence AAOS Evidence-Based Practice Committee Recommendations in Clinical Practice Guidelines Strong • Two or more HIGH quality studies Moderate • One HIGH or 2 MODERATE quality studies Weak • One MODERATE or 1 or more LOW quality studies Consensus • Expert opinion (no studies)*Only used in one circumstance: It pertains to medical interventions that potentially prevent loss of life or limb (catastrophic consequences).