level of evidence

A,B,C,D codes or ratings have been applied to recommendations of the USPSTF & other agencies A: 1) high-quality randomized controlled trial (RCT)* 2) high-quality meta-analysis (quantitative systematic review) B: 1) nonrandomized clinical trial* 2) nonquantitative systematic review 3) lower quality RCT* 4) clinical cohort study 5) case-control study 6) historical control 7) epidemiologic study C: 1) consensus 2) expert opinion D: 1) anecdotal evidence 2) in vitro or animal study * clinical trials are not practical for investigations concerning hazardous environmental substances

General

evidence reliability

References

Siwek J et al, How to write an evidence-based clinical review article, Am Fam Physician 65:25, 2002