The ability to incorporate Evidence-Based Medicine into clinical care requires a basic understanding of the main research designs underlying the published evidence. Some research designs provide a stronger level of evidence than others based on their inherent characteristics. This hierarchy is often shown graphically as a pyramid:

Levels of Evidence

The pyramid is an appropriate shape for this graphic, as it represents the quality of research designs by level, as well as the quantity of each study design in the body of published literature. Systematic reviews (higher quality), for instance, are the most time-intensive articles to write and are therefore rarer (lower quantity) than other types of studies.

More detailed levels of evidence have been developed by the Oxford Centre for Evidence-Based Medicine. They use a numbering scheme ranging from 1a, homogenous systematic reviews of randomized controlled trials, to 5, expert opinion [1]. This system can be especially useful when comparing articles with similar study designs. Equivalent research designs do not always produce results of equal quality.

Rarely does Evidence-Based Medicine draw on research designs lower in the evidence hierarchy than case series, though occasionally nothing but case reports or even bench research may exist on a topic. When making evidence-based decisions for patient care, it is essential to select the highest level research design available for the specific question of interest.