Grading evidence
================

* Holger J. Schünemann
* Dana Best
* Gunn Vist
* Andrew D. Oxman

We agree with Gerald Tevaarwerk that 100-interval scales are useful for many purposes. However, for several reasons (some of which Tevaarwerk has listed and some we discussed1), we disagree that such scales would be useful in the setting of guidelines and recommendations for grading evidence.

First, interval scales suggest a degree of precision that does not exist in the evaluation of quality of evidence; the types of study designs that can be used to determine quality of evidence are limited, which results in few categories. Second, an interval scale for quality of evidence would suggest that we can express quality of evidence in terms of multiples (e.g., “twice as much quality”), but we do not believe that this interpretation is justified. Third, with regard to presentation and practicality, an interval scale would present challenges in the production and dissemination of guidelines. Fourth, with regard to Bayesian reasoning in clinical practice or clinical decision analysis, guideline panels provide guidance to clinicans when they make their recommendations and assign letters, numbers or symbols to those recommendations. Ideally, guideline developers consider clinical decision analysis before they make their recommendations, and we see no use for interval scales that describe the quality of evidence in decision analysis. 

**Holger J. Schünemann** Department of Clinical Epidemiology and Biostatistics McMaster University Hamilton, Ont. **Dana Best** Division of General Pediatrics Children's National Medical Center Washington, DC **Gunn Vist** **Andrew D. Oxman** Department of Health Services Research Norwegian Directorate for Health and Social Welfare Oslo, Norway

## Reference

1.  1. Schünemann HJ, Best D, Vist G, Oxman AD, for the GRADE Working Group. Letters, numbers, symbols and words: how to communicate grades of evidence and recommendations [editorial]. CMAJ 2003;169(7):677-80.
    
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