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Levy Library Blog

Evidence Grades: What Are They and Where Can I Find Them?

by Angelyn Thornton on 2021-06-09T08:00:00-04:00 | Comments


Rachel Pinotti, MLIS (She/her/hers)

Director, Library Education & Research Services​


When seeking answers to clinical questions, it’s important to consider not only what evidence is exists, but also the quality of the evidence. Evidence grades are designed to give healthcare professionals an idea of the strength or weakness of the evidence available to support a recommendation. Read on for an introduction to evidence grading.

Is evidence grading the same as risk of bias assessment?

Evidence grading is related to, but not synonymous with, risk of bias assessment. Assessing the risk of bias occurs at the individual study level, meaning the assessor is looking at the potential for bias, one or more systematic errors, to impact the results of a single study.  In contrast, evidence grades assess the strength of the entire body of evidence on a given topic or question. For instance it might be the case that several studies have been conducted that address the same question. Some of the studies may have utilized observational study design and others may used experimental study design. The available studies likely vary in their sample size and the precision of their estimated effects. Assessing the risk of bias in each of the available studies is a necessary step in order to determine the strength of the body of evidence in order to give a grade. Additionally, while risk of bias assessment makes a determination about the trustworthiness of the results of the study as a whole, evaluators typically assess the evidence and assign grades at the outcome level.  

Are evidence grades standardized across information sources?

Unfortunately not. There are a number of different grading schemas, so it is important to familiarize yourself with the grading schema being utilized in the specific resource you’re consulting. For example, the U.S. Preventive Services Task Force (USPSTF) uses a letter grading schema, consisting of five letter grades (A, B, C, D, or I). In the USPSTF schema, a grade of A indicates that, “The USPSTF recommends the service. There is high certainty that the net benefit is substantial.” Check out the Grade Definitions page on the USPSTF website for complete details on their grade definitions and resulting suggestions for practice. To see the USPSTF in action, check out any of their recommendation statements, for example, the USPSTF final recommendation on Pancreatic Cancer Screening, which has a grade D.

Because the existence of different grading schemas can be confusing, the Grading of Recommendations Assessment, Development and Evaluation (GRADE) working group was formed in 2000 with a goal to develop, “a common, sensible and transparent approach to grading quality (or certainty) of evidence and strength of recommendations.” They released the GRADE schema in the early 2000’s and by 2006, the British Medical Journal (BMJ) identified GRADE as the preferred evidence grading system for clinical practice guidelines published in BMJ.1 The GRADE schema consists of a number (1 or 2) to indicate the strength of a recommendation paired with a letter (A, B, or C) to indicate the quality of the evidence available to support the recommendation. While the GRADE schema has been widely recognized and adopted by a number of professional societies, unfortunately adoption is still far from universal.  

Where will I see evidence grades?

Evidence grades are available in a number of resources, including the USPSTF as discussed above. Clinical practice guidelines are perhaps the most common setting in which you will find evidence grades. They are also frequently available in clinical evidence summary resources. Here are some helpful examples:

Clinical Practice Guidelines

Clinical Evidence Summary Resources


Timing of intervention for AS. Colors correspond to Table 2. Arrows show the decision pathways that result in a recommendation for AVR. Periodic monitoring is indicated for all patients in whom AVR is not yet indicated, including those with asymptomatic (Stage C) and symptomatic (Stage D) AS and those with low-gradient AS (Stage D2 or D3) who do not meet the criteria for intervention. See Section 3.2.4 for choice of valve type (mechanical versus bioprosthetic [TAVI or SAVR]) when AVR is indicated. AS indicates aortic stenosis; AVA, aortic valve area; AVAi, aortic valve area index; AVR, aortic valve replacement; BNP, B-type natriuretic peptide; BP, blood pressure; DSE, dobutamine stress echocardiography ETT, exercise treadmill test; LVEF, left ventricular ejection fraction; ΔPmean, mean systolic pressure gradient between LV and aorta; SAVR, surgical aortic valve replacement; SVI, stroke volume index; TAVI, transcatheter aortic valve implantation; TAVR, transcatheter aortic valve replacement; and Vmax, maximum velocity.


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  1. Guyatt GH, Oxman AD, Vist GE, Kunz R, Falck-Ytter Y, Alonso-Coello P, Schünemann HJ. GRADE: an emerging consensus on rating quality of evidence and strength of recommendations. BMJ. 2008 Apr 24;336(7650):924-6.

  2. Halperin JL, Levine GN, Al-Khatib SM, Birtcher KK, Bozkurt B, Brindis RG, Cigarroa JE, Curtis LH, Fleisher LA, Gentile F, Gidding S. Further evolution of the ACC/AHA clinical practice guideline recommendation classification system: a report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines. Circulation. 2016 Apr 5;133(14):1426-8.

  3. Otto CM, Nishimura RA, Bonow RO, Carabello BA, Erwin III JP, Gentile F, Jneid H, Krieger EV, Mack M, McLeod C, O’Gara PT. 2020 ACC/AHA guideline for the management of patients with valvular heart disease: a report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines. Journal of the American College of Cardiology. 2021 Feb 2;77(4):e25-197.

  4. Homer CJ, Lannon CM, Harbaugh N, Hodgson ES, Marcuse EK, Shiffman RN, Simpson L. Classifying recommendations for clinical practice guidelines. Pediatrics. 2004 Sep 1;114(3):874-7.

  5. Krowchuk DP, Frieden IJ, Mancini AJ, Darrow DH, Blei F, Greene AK, Annam A, Baker CN, Frommelt PC, Hodak A, Pate BM. Clinical practice guideline for the management of infantile hemangiomas. Pediatrics. 2019 Jan 1;143(1).

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