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Evidence Based Medicine: Levels of Evidence & Question Types

Introduction to fundamental concepts of evidence based medicine and practice.

Question Types

As a core component of drafting your clinical questions and PICOs, as well as understanding how to approach your eventual search for evidence and information, it is important to understand the type of question you are asking.  There are a variety of "Question Types" that apply to the EBM process.  Below is a very brief overview of what those question types are, what they represent, and what types of evidence you may look for when asking particular types of questions.

Type of Question


Suggested best type of Study


An evaluation of a therapeutic or preventive intervention

RCT>Cohort>Case Control>Case Series


An evaluation of a test, screening or other assessment such as history or physical exam

Prospective, blind comparison to a gold standard


An evaluation of clinical outcomes over time

Cohort>Case Control>Case Series


An evaluation of a therapeutic, preventive, screening or diagnostic intervention, or a non-therapeutic exposure or behavior

RCT > Cohort > Case Control > Case Series



RCT>Cohort>Case Control>Case Series

Clinical Exam


Prospective, blind comparison to a gold standard



Economic Analysis


Source: WCMC Medical Library -

Levels of Evidence - Pyramid

As you search through databases and information resources, you will find evidence of varying degrees of quality and reliability.  While you will need to critically assess your needs in every situation, the pyramid below highlights the general hierarchy of the various levels of evidence.

In addition to the chart below, the Oxford Centre for Evidence-based Medicine has an excellent page on how certain types of evidence relate to certain types of questions.