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Evidence-Based Practice: Levels of Evidence

Information and resources on EBP including PICO

What are the Levels of Evidence

In order to make medicine more evidence-based, it must be based on the evidence found in research studies with higher quality evidence having more of an impact than lower quality evidence. Evidence is ranked on a hierarchy according to the strength of the results of the clinical trial or research study. The strength of results can be impacted by a variety of factors such as the study design, outcomes, and bias, as well as the results themselves.

Levels of Evidence Resources

Rating System for the Hierarchy of Evidence to Guide Clinical Interventions

Level I - Evidence from a systematic review or meta-analysis of all relevant randomized controlled trials (RCTs)

Level II - Evidence obtained from well-designed RCTs

Level III - Evidence obtained from well-designed controlled trials without randomization

Level IV - Evidence from well-designed case-control and cohort studies

Level V - Evidence from systematic reviews of descriptive and qualitative studies

Level VI - Evidence from single descriptive or qualitative studies

Level VII - Evidence from the opinion of authorities and/or reports of expert committees

Source: Melnyk BM. Implementing the Evidence-Based Practice (EBP) Competencies in Healthcare : A Practical Guide to Improving Quality, Safety, and Outcomes. ; 2016. (Table 1.1, p. 11)

Study Design Tree

Study Design Tree

Q1. What was the aim of the study?

To simply describe a population (PO questions) 

  • Descriptive

To quantify the relationship between factors (PICO questions) 

  • Analytic


Q2. If analytic, was the intervention randomly allocated?


  • RCT


  • Observational study


For observational study the main types will then depend on the timing of the measurement of outcome, so our third question is:


Q3. When were the outcomes determined?

Some time after the exposure or intervention?

  • Cohort study (‘prospective study’)

At the same time as the exposure or intervention?

  • Cross sectional study or survey

Before the exposure was determined?

  • Case-control study (‘retrospective study’ based on recall of the exposure)

Source: Centre for Evidence Based Medicine: Study Designs.