Table 1

Levels of Evidence

Level I

Evidence is based on randomized controlled trials (or meta-analysis of such trials) of adequate size to ensure a low risk of incorporating false-positive or false-negative results.

Level II

Evidence is based on randomized controlled trials that are too small to provide level I evidence. They may show either positive trends that are not statistically significant or no trends and are associated with a high risk of false-negative results.

Level III

Evidence is based on nonrandomized controlled or cohort studies, case series, case-control studies, or cross-sectional studies.

Level IV

Evidence is based on the opinion of respected authorities or expert committees as indicated in published consensus conference proceedings or guidelines.

Level V

Evidence is based on the opinions of those who have written and reviewed the guidelines on the basis of experience, knowledge of the relevant literature, and discussion with peers.

Adapted from Steering Committee on Clinical Practice Guidelines for the Care and Treatment of Breast Cancer: a Canadian consensus document [5].

Becker et al. Allergy, Asthma & Clinical Immunology 2006 2:24   doi:10.1186/1710-1492-2-1-24