Evidence-Based Practice

Oxford Centre for Evidence-Based Medicine (CEBM): Levels of Evidence

The CEBM 'Levels of Evidence' document sets out one approach to systematising this process for different question types. 

While there may be an abundance of research available, it is important to understand how some research designs will produce stronger levels of evidence than others. This has led to the concept of hierarchy of evidence (also known as levels of evidence). The hierarchy "provides a framework for ranking evidence that evaluates healthcare interventions and indicates which studies should be given the most weight in an evaluation where the same question has been examined using different types of study" (Akobeng, 2005, p. 840). The hierarchy is often displayed in the form of a pyramid.

Figure 1: Example of hierarchy of evidence (adapted from Akobeng, 2005)

When looking at the types of research studies in the pyramid, you will notice that the filtered resources are near the top of the pyramid. As you continue down the pyramid, you will find unfiltered resources and background information. Filtered resources have gone through an extensive and systematic evaluation process. Examples include systematic reviews, critically-appraised topics, and critically-appraised articles. When conducting your research, you should try to find the highest quality evidence in systematic reviews (such as integrated research reviews and meta-analyses.)

Figure 2 (adapted from Glover et al., 2006)

As the diagram demonstrates, the top of the pyramid is also the smallest. It may not be possible to answer every clinical question using the highest quality evidence from the top of the pyramid. It may be necessary to move down the pyramid in order to answer your question.

Background information and expert opinion (found in textbooks and other medical books) can be used to better familiarize yourself with a topic when beginning your research.

Next Step: Search the Evidence

Akobeng, A. K. (2005). Understanding randomized controlled trials. Archives of Disease in Childhood, 90, 840-844.

Glover, Jan; Izzo, David; Odato, Karen & Lei Wang. EBM Pyramid. Dartmouth University/Yale University. 2006.