Hierarchy of evidence.

A hierarchy of evidence is a method of categorizing the quality of potential evidence, usually for use in a medical study or making decisions in health care. Experts in evidence-based medicine gather facts and other information to support their ideas and actions but must rank this evidence and focus mainly on the highest-quality material. Different experts have rendered hierarchy charts in different ways. However, a common form is a triangle with seven levels. Weaker evidence occupies the broader bottom levels. Stronger evidence occupies the narrower top levels. Health-care experts start with the best evidence and only move to lower levels if high-quality evidence is lacking.