Grading levels of evidence

Another way of ranking the evidence is to assign a level of evidence to grade the strength of the results measured in a clinical trial or research study. The strength of the evidence is typically based on the reliability (risk of bias) of the study design, the strength of the study outcomes, and applicability to the clinical setting.

Levels of evidence are generally used in clinical practice guidelines and recommendations to allow clinicians to examine the strength of the evidence for a particular course of treatment or action.

The following table has been adapted by Glasziou et al. from the The National Health and Medical Research Council (NHMRC) and The Centre for Evidence-Based Medicine (CEBM) in Oxford. It combines levels of evidence with the type of question and the most appropriate study type.

	Level	Intervention	Diagnosis	Prognosis	Aetiology
Least biased	Ⅰ	Systematic review of Level Ⅱ studies	Systematic review of Level Ⅱ studies	Systematic review of Level Ⅱ studies	Systematic review of Level Ⅱ studies
	Ⅱ	Randomised Controlled Trial	Cross-sectional study among consecutive patients	Inception cohort study	Prospective cohort study
	Ⅲ	One of the Following: non randomised experimental study (e.g. controlled pre- and post-test intervention study) Comparative (observational) study with a concurrent control group (e.g. cohort study, case-control study)	One of the following: Cross-sectional study among non-consecutive patients Diagnostic case-control study	One of the following: Untreated control patients in a randomised controlled trial Retrospectively assembled cohort study	One of the following: Retrospective cohort study Case-control study
Most biased	Ⅳ	Case series	Case Series	Case series, or a cohort study of patients at different stages of disease	A cross-sectional study