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Evidence Based Medicine (EBM)

This guide is designed to walk you through the Evidence Based Medicine process: the elements of a well-formulated clinical question, types of studies, and the key critical appraisal questions that help determine the validity of evidence.

What type of question?

The study designs best suited for answering your question will depend on the type of question being asked.

Common question types:

  • Therapy: Does this treatment do more good than harm and is it worth the effort and cost?
  • Diagnosis: Which diagnostic test is most accurate in confirming or excluding a diagnosis, based on a consideration of specificity, sensitivity, likelihood ratios, expense, safety, etc.? Diagnostic questions usually compare the accuracy of an 'index test' against a reference test, or 'gold standard'.
  • Prognosis: How do we estimate the patient's likely clinical course over time and anticipate likely complications of disease?
  • Etiology/Harm: What causes the health care problem or makes it more likely? E.g. Do tanning beds increase the risk of melanoma? Does obesity lead to increased risk of cardiovascular disease? What behavioural risk factors have been linked to lung cancer?
  • Prevention: How can we reduce the chance of disease? What interventions have proven effective in modifying risk factors? (e.g. diagnostic tests, therapeutic programs, health promotion activities).
  • Qualitative: How is the person experiencing what is happening to them? What are their perceptions, beliefs, attitudes?


Best designs for specific question types*

Type of Question  Best Type of Study
   Therapy RCT -> cohort -> case control -> case series
   Diagnosis                 prospective, blind comparison to a gold standard (e.g. consecutive cohort study) ⇒ cross-sectional study
   Etiology/Harm RCT -> cohort -> case control -> case series
   Prognosis Cohort study -> case control -> case series
   Prevention RCT -> cohort study -> case control -> case series


*For all types of clinical questions, a systematic review of all the available studies is better than an individual study.