Table 2.

Levels of Scientific Evidence

Level	Description
I	Evidence based on randomized controlled clinical trials (or meta-analysis of such trials) of adequate size to ensure a low risk of incorporating false-positive or false-negative results.
II	Evidence based on randomized controlled trials 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.
III	Evidence based on nonrandomized, controlled, or cohort studies; case series; case-controlled studies; or cross-sectional studies.
IV	Evidence based on the opinion of respected authorities or expert committees as indicated in published consensus conferences or guidelines.
V	Evidence that expresses the opinion of those individuals who have written and reviewed this guideline, based on experience, knowledge of the relevant literature, and discussions with peers.

Sources: Adapted from Sackett DL. Rules of evidence and clinical recommendation on the use of antithrombotic agents. Chest. 1989 95(suppl 2):2S-4S;1 and U.S. Preventive Health Services Task Force. Guide to Clinical Preventive Services. 2^nd ed. Baltimore, MD: Williams and Wilkins; 1996.²