[The values of different study designs on the levels of evidence: a descriptive analysis of the researches published in four general medical journals in 2009].

Objective: To discuss the levels of evidence provided by different study designs.

Methods: Websites of N Engl J Med, JAMA, Lancet, and BMJ were accessed to identify research articles (systematic review and meta-analysis included) published in 2009. A standardized data collection form was established using Epidata 3.