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Evidence Based Medicine

Evidence based medicine (EBM) is the conscientious, explicit, judicious and reasonable use of modern, best evidence in making decisions about the care of individual patients.

EBM Pyramid

ebm pyramid

Case Series and Case Reports:
  • Collections of reports on the treatment of individual patients or a report on a single patient.
  •  No control groups with which to compare outcomes, so limited statistical validity.

Case Control Studies:

  •  Patients who already have a specific condition are compared with people without the condition. Researcher looks back to identify factors or exposures possibly associated with the condition, often relying on medical records and patient recall.
  •  Less reliable because showing a statistical relationship does not mean than one factor necessarily caused the other. 
  •  Starts with patients who already have the outcome and looks backwards to possible exposures.

Cohort Studies:

  •  Take a large population who are already taking a particular treatment or have an exposure, follow them forward over time, and then compare for outcomes with a similar group that has not been affected by the treatment or exposure.
  •  Observational and not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study. 
  • Starts with the exposure and follows patients forward to an outcome.

Randomized, Controlled Clinical Trials:

  •  Carefully planned projects that introduce a treatment or exposure to study its effect on patients.
  •  Include methodologies that reduce the potential for bias (randomization and blinding) and allow for comparison between intervention and control groups. 
  •  Is an experiment and can provide sound evidence of cause and effect.  
  •  Randomly assigns exposures and then follows patients forward to an outcome.

Systematic Reviews:

  • Usually focus on a clinical topic and answer a specific question. An extensive literature search is conducted to identify studies with sound methodology.
  • The studies are reviewed, assessed, and the results summarized according to the predetermined criteria of the review question. 


  •  Thoroughly examines a number of valid studies on a topic and combines the results using accepted statistical methodology to report the results as if it were one large study. 
  •  The Cochrane Collaboration has done a lot of work in the areas of systematic reviews and meta-analysis.