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Evidence-Based Practice: Study Design Hierarchy

Type of Question

Diagnosis Question:
Does this diagnostic test accurately distinguish between patients who do and do not have a specific disorder?1
Therapy Question:
What effect does an intervention have on particular patient groups? Does it do more good than harm? Determination of clinical and cost effectiveness.
Prognosis Question:
What is the probable clinical course of this patient over time? What main prognostic factor am I considering? 3  
Harm/Etiology Question:
Does this exposure/intervention cause a harmful effect on patients? What are the causes/risk factors for disease?2


Information adapted from the following sources:
1. Sackett, DL, et al. Evidence Based Medicine: How to Practice and Teach EBM. 2nd ed. Edinburgh: Churchill Livingstone, 2000. BHSL Call Number: WB 102 E933 2000
2. Tutorial: How To Form and Answerable Clinical Question. (2006) Cincinnati Children's.

6S Hierarchy of Pre-Appraised Evidence

DiCenso, A., Bayley, L., & Haynes, R. B. (2009). Accessing pre-appraised evidence: fine-tuning the 5S model into a 6S modelEvidence based nursing,12(4), 99-101.

Type of Question Determines Type of Study

Select the best study type to answer your clinical question:

Type of Question

Best Study Type


RCT > Cohort > Case Control > Case Series


Prospective, Blind Comparison to Gold Standard


Cohort > Case Control > Case Series


RCT > Cohort > Case Control > Case Series

Questions answered best by RCTs may also be answered by systematic reviews and meta-analyses

Study Design Hierarchy

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