Table 2. Rating System Used to Rate Level of Evidence and Strength of Recommendation
Recommendations are based on scientific evidence and expert opinion. Each recommended statement includes a Roman numeral (I, II, or III) representing the level of the evidence that supports the recommendation and a letter (A, B, or C) representing the strength of the recommendation.
CLASS |
|
|---|---|
I |
Evidence and/or general agreement that a given diagnostic evaluation, procedure, or treatment is beneficial, useful, and effective. |
II |
Conflicting evidence and/or a divergence of opinion about the usefulness and efficacy of a diagnostic evaluation, procedure, or treatment. |
IIa |
Weight of evidence and/or opinion is in favor of usefulness and efficacy. |
IIb |
Usefulness and efficacy are less well established by evidence and/or opinion. |
III |
Conditions for which there is evidence and/or general agreement that a diagnostic evaluation, procedure, or treatment is not useful and effective or if it in some cases may be harmful. |
Level |
|
|---|---|
A |
Data derived from multiple randomized clinical trials, meta-analyses, or equivalent. |
B |
Data derived from a single randomized trial, nonrandomized studies, or equivalent. |
C |
Consensus opinion of experts, case studies, or standard of care. |
Last update: November 6, 2019
Adapted from the American College of Cardiology and the American Heart Association Practice Guidelines (American College of Cardiology Foundation and American Heart Association, 2010); (Shiffman, 2003).
In some situations, randomized clinical trials with an existing standard-of-care arm cannot ethically or practicably be conducted. The US Food and Drug Administration (FDA) has suggested alternative study designs, including historical controls or immediate versus deferred placebo-controlled trials. For additional information, visit the Alternative Methods section of the FDA website. In those instances for which there was a single predetermined, FDA-approved equivalency established, panel members consider the evidence as equivalent to a randomized, controlled trial for level A or B.
NOTE: URL verified 4/30/25
Shiffman RN, Shekelle P, Overhage JM, Slutsky J, Grimshaw J, Deshpande AM.
Standardized reporting of clinical practice guidelines: a proposal from the Conference on Guideline Standardization. Ann Intern Med. 2003;139(6):493-498.
Last Update: November 6, 2019 Last Review: January 15, 2025
