Levels of Evidence

Level 1 Evidence Level 1 evidence can come in one of 2 forms: a randomized clinical trial, which is associated with low study errors (low P value, confidence interval which excludes 1, or a ‘‘negative’’ result associated with a power of over 80%), or a meta-analysis. A meta-analysis refers to a study that statistically synthesizes the results of numerous ‘‘underpowered’’ randomized clinical trials and increases the confidence in an overall summary result. Unfortunately, limitations of meta-analyses include heterogeneity and publication bias. The Optic Neuritis Treatment Trial is an example of a randomized clinical trial which contains level 1 evidence.