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Evidence Based Practice: Step 3: Study designs

When answering a clinical question or treating a patient, it's important to find the best level of evidence. The video below explains how to use three different level of evidence tools - the Evidence Based Medicine Pyramid, the NHMRC Evidence Hierarchy, and the Oxford Centre for Evidence Based Medicine's Level of Evidence Table.



For most types of clinical questions, a well-done systematic review or meta-analysis of all available studies is the best level of evidence. A comprehensive systematic review will compare multiple randomised controlled trials that have studied the same treatment and aggregate their results. 

The list below shows which study types are the most appropriate to answer different clinical question types. 

  • Therapy: randomised controlled trials (RCTs)
  • Diagnosis: independent studies > blind comparison to a gold standard
  • Prognosis: cohort studies > case control studies > case series
  • Aetiology: RCTs > cohort studies > case control studies > case series
  • Prevention: RCTs > cohort studies > case control studies
  • Cost-effectiveness: economic analyses
  • Quality of life: qualitative studies

For more information about study designs, see 'How to read a paper' by Trish Greenhalgh.


Step 2: Use PICO to
structure the question


Step 4: Plan a 
literature search