Now that you have developed an answerable research question, it is time to search for evidence.  Before you start your search, figure out what type of studies are most appropriate to answer your question and what level of evidence they provide.

A cornerstone of EBM is the hierarchical system of classifying evidence known as the levels of evidence. Levels of evidence (LOE) are used to describe the strength of the results measured in a clinical trial or research study. Evidence is ranked based on its reliability. LOE models review data based on the elements of study design, quality, and consistency of evidence.

Start at the top of the pyramid for the most reliable evidence, keeping in mind that research may not yet exist or may not be feasible for some questions.  

 Levels of Evidence Pyramid by Krystal Bullers is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

A frequently used system in medicine is from the Oxford Center for Evidence-Based Medicine. They have developed the below Levels of Evidence table for a more nuanced approach to evaluating evidence based on the type of question you are asking. In this model, Level 1 evidence is the most reliable for decision making. For example, therapy questions are best answered by systematic reviews of randomized controlled trials. 

It is intended to be used in conjunction with Introductory Document and Background Document. Links are provided below the table.

OCEBM Levels of Evidence Working Group*. “The Oxford Levels of Evidence 2”.
Oxford Centre for Evidence-Based Medicine. https://www.cebm.ox.ac.uk/resources/levels-of-evidence/ocebm-levels-of-evidence