Levels of Evidence (LOE)

Level 1: Evidence is based on RCTs (or meta-analysis of such trials) of adequate size to ensure a low risk of incorporating false-positive or false-negative results.

Level 2: Evidence is based on RCTs that are too small to provide level I evidence. These 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 3: Evidence is based on nonrandomized, controlled, or cohort studies; case series; case-controlled or cross-sectional studies.

Level 4: Evidence is based on the opinion of respected authorities or that of expert committees as indicated in published consensus conferences or guidelines.

Level 5: Evidence expresses the opinion of those individuals who have written and reviewed these guidelines based on their experience, knowledge of the relevant literature, and discussion with their peers.

Steering Committee on Clinical Practice Guidelines for the Care and Treatment of Breast Cancer (1998)

Advanced LOE example: http://www.cebm.net/index.aspx?o=1025

What is the adequate size to make a RCT a Level One?

“There is no hard and fast rule for the size of n, but you can generalize and say that EACH group within the RCT design would need to have at least 30-60 participants. That means if there was just one experimental group plus a control, the n would need to be at least 60-120. If there were two experimental groups plus a control, the n would need to be at least 90-180. In the field of rehabilitation, very few RCT are large enough to be Level One.”

(Marian Minor, personal communication, Oct. 15, 2006)