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CATWalk: A Guide to Critically Appraised Topics

Ask a Clinical Question

Asking a well-built clinical question is usually harder than it may at first appear. However, once you master the art of asking focused and relevant questions, finding an answer will be much more straightforward.

The major components to include in any question are Population/Patient, Intervention, Comparison (optional), and Outcome. You may often see these components represented by the acronym PICO.

This breakdown of components will ensure that you focus your question and include the key variables which you can then translate into a search for evidence on that topic.

PICO Ask Yourself: Example:
Population How would I describe a group of patients similar to mine? Children with chronic otitis media
Intervention Which main intervention, prognostic factor, or exposure am I considering? Antibiotic treatment
Comparison What is the main alternative to compare with the intervention? Non-treatment
Outcome What can I hope to accomplish, measure, improve or affect? Reduce symptoms

Chart based on: Duke University Medical Center Library's "Overview of EBM and Anatomy of a Question".


There are four main domains into which your clinical question may fall.  In order to determine how to search for evidence, you must decide which type of question you are asking.

  • Therapy - "how to select treatments to offer our patients that do more good than harm and that are worth the efforts and costs of using them." 
  • Diagnosis - "how to select and interpret diagnostic tests, in order to confirm or exclude a diagnosis, based on considering their precision, accuracy, acceptability, safety, expense, etc." 
  • Prognosis - "how to estimate our patient's likely clinical course over time and anticipate likely complications of the disorder."
  • Etiology - "how to identify causes for disease (including its iatrogenic forms)."
  • Definitions taken from Straus et al. Evidence-Based Medicine: How to Practice and Teach EBM (London: Churchill Livingstone, 2005): 20.


  • Ask Yourself: Select From:
    In which domain should the clinical question be placed? Therapy, Diagnosis, Prognosis, or Causation/Harm
    What type of study would best answer the clinical question? Systematic reviews, randomized controlled trials, cohort or case-control studies, etc.