Table 1.

Levels of evidence.

I	Large randomized clinical control trials with clear cut results or systematic review of these articles
II	Small randomized clinical control trials with unclear results or systematic reviews of these articles
III	Prospective cohort and case-control studies or systematic reviews of these articles
IV	Historical cohort or case-control studies of systematic reviews of these articles
V	Case series, studies with no controls or Expert opinion

(Adapted from: Sackett DL. Rules of evidence and clinical recommendations on the use of antithrombotic agents. Chest 1989; 95:2S–4S).