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School of Nursing: Evidence-Based Practice: Study Types & Levels

An overview of evidence-based practice resources for Nursing students.

Levels of Evidence

The level of evidence can be defined by the type of question. Levels of evidence for a "Therapy" question are different from the levels of evidence for an "Etiology" question. Please review the following from the Centre for Evidence-Based Medicine at Oxford:

Level Description

Levels of Evidence in Nursing Research

I Evidence from a systematic review or meta-analysis of all relevant RCTs (randomized controlled trial) or evidence-based clinical practice guidelines based on systematic reviews of RCTs or three or more RCTs of good quality that have similar results.
II Evidence obtained from at least one well-designed RCT (e.g. large multi-site RCT).

Evidence obtained from well-designed controlled trials without randomization (i.e. quasi-experimental).

IV Evidence from well-designed case-control or cohort studies.
V Evidence from systematic reviews of descriptive and qualitative studies (meta-synthesis).
VI Evidence from a single descriptive or qualitative study.

Evidence from the opinion of authorities and/or reports of expert committees.

The table above is based on the following: Ackley, B. J., Swan, B. A., Ladwig, G., & Tucker, S. (2008). Evidence-based nursing care guidelines: Medical-surgical interventions. (p. 7)St. Louis, MO: Mosby Elsevier.

Types of Clinical Questions

Therapy - What is the treatment for a disease?

Diagnosis - power of a test to differentiate between those with and without a disease?

Prognosis - a patient’s likely course over time due to factors other than interventions?

Etiology/Harm - effects of potentially harmful agents on the patient?

Study Types to Answer Various Questions

Type of Question Suggested Best Type of Study
Therapy RCT > prospective cohort
Diagnosis Prospective, blind comparison to a gold standard
Etiology/Harm RCT > cohort > case control > case series
Prognosis cohort study > case control > case series
Prevention RCT > cohort study > case control > case series
Clinical Exam prospective, blind comparison to gold standard
Cost economic analysis