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Different research study methodologies are prone to different types and amounts of error. Those that are the least error prone are considered the very best evidence. This assumes, of course, that the study is properly executed.
Evidence-based practice has ranked these methodologies into Levels of Evidence, with Level One evidence being the best. The higher levels of evidence reflect studies which include less bias and tend to utilize control groups. However, due to the time it takes to conduct a systematic review or meta-analysis, fewer of those studies exist. In time, hopefully, that will be remedied.
The definitions of Levels of Evidence may vary by author, publication, or source. Check with your instructor to see what definitions are being used for the Levels of Evidence in your class. However, regardless of whether your definition uses 5 or 7 levels of evidence, the value of the evidence provided by the different types of publications and the relative order of the methodologies remains unchanged.
Rating System for Levels of Evidence (LoBiondo, 2010)
LoBiondo-Wood, G.Haber, J. (2010). Nursing research: Methods and critical appraisal for evidence-based practice St. Louis, Mo.: Mosby/Elsevier.
Melnyk, B. M., & Fineout-Overholt, E. (2010). Evidence-based practice in nursing & healthcare: A guide to best practice. Philadelphia: Lippincott Williams & Wilkins.
