level of evidence

Introduction

A,B,C,D codes or ratings have been applied to recommendations of the USPSTF & other agencies

A:

• high-quality randomized controlled trial (RCT)*
• high-quality meta-analysis (quantitative systematic review)

B:

• nonrandomized clinical trial*
• nonquantitative systematic review
• lower quality RCT*
• clinical cohort study
• case-control study
• historical control
• epidemiologic study

C:

• consensus
• expert opinion

D:

• anecdotal evidence
• in vitro or animal study

* clinical trials are not practical for investigations concerning hazardous environmental substances

More general terms

• evidence