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Evidence-Based Research: Types of Questions/Studies

Levels of Evidence


                                       Image of hierarchy of scientific evidence

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See the Glossary (PDF) provided by the University of Arizona College of Pharmacy for definitions of study designs. 

The Evidence-Based Pyramid is divided into levels. Each ascending level represents a different type of study design and corresponds to increasing rigor, quality, and reliability of the evidence. The levels from the bottom to the top are: case reports, opinion papers, & letters; animal trials; cross-sectional studies; case-control studies; cohort studies; randomized controlled trials; and metanalyses & systematic reviews. 

Types of Questions

Therapy: Questions about the effectiveness of interventions in improving outcomes in sick patients/patients suffering from some condition. 

  • Example: Is Tamiflu more effective than over-the-counter medications in shortening the recovery period from the flu? 
  • Ideal types of studies: Randomized Controlled Trial

Prevention: Questions about the effectiveness of an intervention or exposure in preventing morbidity and mortality. 

  • Example: Are pressure-reducing mattresses more effective than patient repositioning in preventing pressure ulcers amongst elderly nursing home residents? 
  • Ideal types of studies: Randomized Controlled Trial; Cohort Study; Case-Control

Diagnosis: Questions about the ability of a test or procedure to differentiate between those with and without a condition or disease. 

  • Example: Is ultrasound more effective than a mammogram in detecting breast cancer in symptomatic younger women? 
  • Ideal types of studies: Prospective, blind controlled trial comparison to the gold standard

Prognosis: Questions about the probable cause of a patient's disease or the likelihood that he or she will develop an illness. 

  • Example: Do premature babies have an increased risk of developing asthma as adolescents? 
  • Ideal types of studies: Cohort Study; Case-Control; Case Series/Case Report

Etiology/Harm: Questions about the harmful effect of an intervention or exposure on a patient. 

  • Example: Are male adolescents who consume energy drinks more prone to migraines than male adolescents who do not consume energy drinks? 
  • Ideal types of studies: Randomized Controlled Trial; Cohort Study; Case-Control