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 |
|---|---|
| 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). |
| III |
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. |
| VII |
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.
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?
| 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 |
