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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

Description

Suggested best type of Study

Therapy

An evaluation of a therapeutic or preventive intervention

RCT>Cohort>Case Control>Case Series

Diagnosis

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

Prospective, blind comparison to a gold standard

Prognosis

An evaluation of clinical outcomes over time

Cohort>Case Control>Case Series

Etiology/Harm

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

RCT > Cohort > Case Control > Case Series

Prevention

 

RCT>Cohort>Case Control>Case Series

Clinical Exam

 

Prospective, blind comparison to a gold standard

Cost

 

Economic Analysis

 

Source: WCMC Medical Library - http://med.cornell.libguides.com/content.php?pid=295507&sid=5600835

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.